by pinning to macos-14. I believe this problem started when Github
upgraded to macOS 15:

> The macos-latest label will migrate to macOS 15 beginning August 4, 2025

The error when system libraries are disabled:

    ./libtool: line 1871: /Applications/Xcode_15.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar: No such file or directory

The error when system libraries are enabled:

    make: /Applications/Xcode_15.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar: No such file or directory

(cherry picked from commit 1dac08d)

Drop testing system libraries for Ruby 3.1 because devkit is built
with a version of GCC that isn't compatible with modern msys2.

The error, on both syslib-enabled and -disabled Ruby 3.1 builds:

    pacman.exe -Sy --noconfirm --noprogressbar --needed --disable-download-timeout  mingw-w64-ucrt-x86_64-libxml2 mingw-w64-ucrt-x86_64-libxslt
    looking for conflicting packages...
    error: failed to prepare transaction (could not satisfy dependencies)
    :: installing mingw-w64-ucrt-x86_64-gcc-libs (15.2.0-8) breaks dependency 'mingw-w64-ucrt-x86_64-gcc-libs=14.2.0-3' required by mingw-w64-ucrt-x86_64-gcc
    Error: Command failed: pacman.exe -Sy --noconfirm --noprogressbar --needed --disable-download-timeout  mingw-w64-ucrt-x86_64-libxml2 mingw-w64-ucrt-x86_64-libxslt

(cherry picked from commit ae334cd)

Related to (and partial reversion of) ae334cd.

Modern msys has diverged too far from DevKit to make it worth my time
to continue to support source builds.

(cherry picked from commit 9557c54)

(cherry picked from commit 71e4a21)

https://savannah.gnu.org/news/?id=10703

Fixes #3519

(cherry picked from commit 5630f82)

**What problem is this PR intended to solve?**

https://savannah.gnu.org/news/?id=10703

Fix #3543 on the v1.18.x branch
Related to #3519

This is a backport of #3520

Also apply a few CI improvements to try to get green, generally.

Also, drop the merged upstream patches previously applied to v2.13.8.

**What problem is this PR intended to solve?**

https://gitlab.gnome.org/GNOME/libxml2/-/releases

Also, drop the merged upstream patches previously applied to v2.13.8.