Don't try and build for Ruby < 2.6 on macos ARM

It doesn't work. Using macos-13 will build on intel machines and will
work, however. This is the recommendation from the setup-ruby action.

Author: KJ Tsanaktsidis

.github/workflows/test.yml

@@ -16,6 +16,12 @@ jobs:
       matrix:
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
         os: [ubuntu-latest, macos-latest]
+        exclude:
+          - { os: macos-latest, ruby: '2.4' }
+          - { os: macos-latest, ruby: '2.5' }
+        include:
+          - { os: macos-13, ruby: '2.4' }
+          - { os: macos-13, ruby: '2.5' }
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4