sagemathgh-36999: Rename `install-requires.txt` to `version_requirements.txt`
    
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Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes sagemath#1234" use "Introduce new method to
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As discussed in sagemath#36982:
- the name "install-requires"  has become outdated with the transition
to the new names set by the `pyproject.toml` format (see https://setupto
ols.pypa.io/en/latest/userguide/dependency_management.html#declaring-
required-dependency, compare the tabs "pyproject.toml" vs. "setup.cfg")
- this will help with sagemath#35890, as
GitHub will be able to just read the `version_requirements.txt` files

sagemath#36982 (comment)

Notes for reviewers:
- **This is a simple, limited-scope improvement and not intended as a
long-term commitment to the format of the files in build/pkgs/....**
- PRs sagemath#37430, sagemath#37350, sagemath#36740 remove direct access to build/pkgs in favor
of using the sage_bootstrap API (sage --package).

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example "Fixes sagemath#12345". -->
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### 📝 Checklist

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- [x] 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: ...
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- Depends on sagemath#37401

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URL: sagemath#36999
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik, Kwankyu Lee, Nathan Dunfield

sagemathgh-36999: Rename `install-requires.txt` to `version_requirements.txt`
    
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes sagemath#1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->

As discussed in sagemath#36982:
- the name "install-requires"  has become outdated with the transition
to the new names set by the `pyproject.toml` format (see https://setupto
ols.pypa.io/en/latest/userguide/dependency_management.html#declaring-
required-dependency, compare the tabs "pyproject.toml" vs. "setup.cfg")
- this will help with sagemath#35890, as
GitHub will be able to just read the `version_requirements.txt` files

sagemath#36982 (comment)

Notes for reviewers:
- **This is a simple, limited-scope improvement and not intended as a
long-term commitment to the format of the files in build/pkgs/....**
- PRs sagemath#37430, sagemath#37350, sagemath#36740 remove direct access to build/pkgs in favor
of using the sage_bootstrap API (sage --package).

<!-- Why is this change required? What problem does it solve? -->
<!-- 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. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [x] The title is concise, informative, and self-explanatory.
- [x] 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: ...
-->
- Depends on sagemath#37401

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: sagemath#36999
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik, Kwankyu Lee, Nathan Dunfield