sagemathgh-35810: Drop support for GCC < 8.4, drop testing of `debian-buster` and `fedora-29`
    
<!-- Please provide a concise, informative and self-explanatory title.
-->
<!-- Don't put issue numbers in the title. Put it in the Description
below. -->
<!-- For example, instead of "Fixes sagemath#12345", use "Add a new method to
multiply two integers" -->

### 📚 Description

<!-- Describe your changes here in detail. -->
<!-- Why is this change required? What problem does it solve? -->
Prerequisite of:
- sagemath#34816
- sagemath#35703
- sagemath#35801

See also:
- sagemath#32074

Instead of fully dropping `debian-buster` (LTS until 2024-06) in the CI,
we replace it by `debian-buster-gcc_spkg` to verify that users on this
platform have the recourse of installing a newer GCC.

We also remove `linuxmint-19.3-gcc_8-python3.8` and change
`fedora-30-python3.8` to `fedora-30` (which builds python from spkg), to
prevent a merge conflict with
sagemath#35404

We also add `debian-trixie` (= current testing).

<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes sagemath#12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->

- [x] The title is concise, informative, and self-explanatory.
- [ ] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- sagemath#12345: short description why this is a dependency
- sagemath#34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: sagemath#35810
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik

sagemathgh-35810: Drop support for GCC < 8.4, drop testing of `debian-buster` and `fedora-29`
    
<!-- Please provide a concise, informative and self-explanatory title.
-->
<!-- Don't put issue numbers in the title. Put it in the Description
below. -->
<!-- For example, instead of "Fixes sagemath#12345", use "Add a new method to
multiply two integers" -->

### 📚 Description

<!-- Describe your changes here in detail. -->
<!-- Why is this change required? What problem does it solve? -->
Prerequisite of:
- sagemath#34816
- sagemath#35703
- sagemath#35801

See also:
- sagemath#32074

Instead of fully dropping `debian-buster` (LTS until 2024-06) in the CI,
we replace it by `debian-buster-gcc_spkg` to verify that users on this
platform have the recourse of installing a newer GCC.

We also remove `linuxmint-19.3-gcc_8-python3.8` and change
`fedora-30-python3.8` to `fedora-30` (which builds python from spkg), to
prevent a merge conflict with
sagemath#35404

We also add `debian-trixie` (= current testing).

<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes sagemath#12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->

- [x] The title is concise, informative, and self-explanatory.
- [ ] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- sagemath#12345: short description why this is a dependency
- sagemath#34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: sagemath#35810
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik

sagemathgh-37514: `MatrixSpace`: Support constructing `Hom(CombinatorialFreeModule)`
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->
The `FreeModule` constructor was recently generalized so that it can
create `CombinatorialFreeModule` objects. In particular,
```
sage: ZZ^3
Ambient free module of rank 3 over the principal ideal domain Integer
Ring
sage: ZZ^['a','b','c']
Free module generated by {'a', 'b', 'c'} over Integer Ring
```

Here as a follow-up, we extend `MatrixSpace` in a similar way: When
lists or enumerated sets of row indices and column indices are given, it
constructs `CombinatorialFreeModule`s whose bases are indexed by these
index sets, and then returns the homset. In particular,
```
sage: ZZ^(2, 3)
Full MatrixSpace of 2 by 3 dense matrices over Integer Ring
sage: ZZ^(['x','y'], ['a', 'b', 'c'])
Set of Morphisms from Free module generated by {'a', 'b', 'c'} over
Integer Ring to Free module generated by {'x', 'y'} over Integer Ring in
Category of finite dimensional modules with basis over Integer Ring
```

Follow-up PRs:
- extend `matrix` likewise; guiding application: adjacency / incidence
"matrices" of graphs, where rows/columns are indexed by nodes and edges
- see also sagemath#35801

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37514
Reported by: Matthias Köppe
Reviewer(s): Frédéric Chapoton, gmou3, Matthias Köppe, Travis Scrimshaw

sagemathgh-37514: `MatrixSpace`: Support constructing `Hom(CombinatorialFreeModule)`
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->
The `FreeModule` constructor was recently generalized so that it can
create `CombinatorialFreeModule` objects. In particular,
```
sage: ZZ^3
Ambient free module of rank 3 over the principal ideal domain Integer
Ring
sage: ZZ^['a','b','c']
Free module generated by {'a', 'b', 'c'} over Integer Ring
```

Here as a follow-up, we extend `MatrixSpace` in a similar way: When
lists or enumerated sets of row indices and column indices are given, it
constructs `CombinatorialFreeModule`s whose bases are indexed by these
index sets, and then returns the homset. In particular,
```
sage: ZZ^(2, 3)
Full MatrixSpace of 2 by 3 dense matrices over Integer Ring
sage: ZZ^(['x','y'], ['a', 'b', 'c'])
Set of Morphisms from Free module generated by {'a', 'b', 'c'} over
Integer Ring to Free module generated by {'x', 'y'} over Integer Ring in
Category of finite dimensional modules with basis over Integer Ring
```

Follow-up PRs:
- extend `matrix` likewise; guiding application: adjacency / incidence
"matrices" of graphs, where rows/columns are indexed by nodes and edges
- see also sagemath#35801

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37514
Reported by: Matthias Köppe
Reviewer(s): Frédéric Chapoton, gmou3, Matthias Köppe, Travis Scrimshaw