Fix errors building Ruby 3.1 on windows

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)

Author: flavorjones

.github/workflows/ci.yml

@@ -328,6 +328,9 @@ jobs:
       matrix:
         sys: ["enable", "disable"]
         ruby: ${{ fromJSON(needs.ruby_versions.outputs.setup_ruby_win) }}
+        exclude:
+          - sys: "enable"
+            ruby: "3.1" # because Ruby 3.1 devkit is built with a version of GCC too old for modern msys2 2025-08
     runs-on: windows-2022
     steps:
       - name: configure git crlf
@@ -340,7 +343,7 @@ jobs:
       - uses: ruby/setup-ruby-pkgs@v1
         with:
           ruby-version: "${{matrix.ruby}}"
-          mingw: "libxml2 libxslt"
+          mingw: ${{ matrix.ruby == '3.1' && ' ' || 'libxml2 libxslt' }} # '' is falsey, ' ' is truthy
           bundler-cache: true
           bundler: latest
       - uses: actions/cache@v4
@@ -551,6 +554,9 @@ jobs:
       matrix:
         sys: ["enable", "disable"]
         ruby: ${{ fromJSON(needs.ruby_versions.outputs.setup_ruby_win) }}
+        exclude:
+          - sys: "enable"
+            ruby: "3.1" # because Ruby 3.1 devkit is built with a version of GCC too old for modern msys2 2025-08
     runs-on: windows-2022
     steps:
       - uses: actions/checkout@v4
@@ -559,7 +565,7 @@ jobs:
       - uses: ruby/setup-ruby-pkgs@v1
         with:
           ruby-version: "${{matrix.ruby}}"
-          mingw: "libxml2 libxslt"
+          mingw: ${{ matrix.ruby == '3.1' && ' ' || 'libxml2 libxslt' }} # '' is falsey, ' ' is truthy
       - uses: actions/download-artifact@v4
         with:
           name: generic-gem