Fix CI by explicit use of macos-13 instead of -latest. (#131)

macos-latest no longer supports Ruby < 2.7 due to architectural changes.

Author: ioquatix

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
-        os: [ ubuntu-latest, macos-latest ]
+        os: [ ubuntu-latest, macos-13 ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4