Currently this crate maintains the `main` branch (future 1.0) alongside
`libc-0.2` (current stable release). PRs are made against `main` then
cherry picked as applicable to `libc-0.2`.

Usually this flow works okay, but there is a substantial difference in
minimum supported versions: libc-0.2 is tested down to 1.19 and main is
tested with 1.63. This means that supported features differ quite a bit
and as a result, cherry picks get conflict-heavy (e.g. `repr(align)`,
`union`, and `const fn` cannot be used on `libc-0.2`).

In order to make it easier to keep these branches in sync and get us
close to a 1.0 release, raise the MSRV on `libc-0.2` to 1.63. This means
both branches can run the exact same tests.

Future Changes
==============

This still does not establish a MSRV policy, which has been discussed at
great length in [1]. For the purpose of unsticking us this selects 1.63
as the MSRV, which is the version currently available on Debian stable
(a commonly requested reference point in [1], and about the oldest
specific version mentioned).

This is a documentation-only change to keep things simple, cleanup can
follow. Further increases and official policy are not precluded.

History
=======

An attempt to raise the MSRV to 1.57 in 2022 was approved at one point
[2], but never merged due to various failures. Making this a
documentation-only change hopes to avoid this problem.

I brought up a 0.3 release to increase versions in [3], but consensus
there was that we should be able to increase the MSRV in the existing
0.2 release without a semver-breaking change.

Link: rust-lang/libs-team#72 [1]
Link: #2845 [2]
Link: rust-lang/libs-team#463 [3]

With the change to MSRV in [1], update Cargo.toml `rust-version` to
match.

[1]: #4040

(backport <#4050>)
(cherry picked from commit 4b21887)

Since the increase to MSRV in [1], we no longer need to test with old
versions of Rust.

This makes CI identical to `main`, with the exception of branch
configuration.

[1]: #4040

This workflow is redundant now that Bors is no longer used. Remove it.

The derived implementations were causing a CI failure that didn't show
up before. Manually implement `PartialEq` and `Debug` to work around
this.

chore: release v0.2.163

[0.2] Document the MSRV of the stable channel as 1.63

[0.2] Set `rust-version` to 1.63

[0.2] ci: Remove tests with rust < 1.63