CI: use macos-12 -> macos-13

The macOS 12 Actions runner image has been removed:
actions/runner-images#10721

Author: dentarg

.github/workflows/test.yml

@@ -105,7 +105,7 @@ jobs:
           # Ubuntu
           - { os: ubuntu-22.04, ruby: 3.1 }
           # macOS
-          - { os: macos-12,     ruby: 3.1 }
+          - { os: macos-13,     ruby: 3.1 }
           # Windows
           - { os: windows-2019, ruby: 3.1 }
           - { os: windows-2022, ruby: 3.1 }