CI: use macos-13 for Ruby 2.5

Author: olleolleolle

.github/workflows/ci.yaml

@@ -17,14 +17,15 @@ jobs:
         os: [ubuntu]
         ruby: ['2.5', '2.6', '2.7', '3.0', '3.1', '3.2']
         include:
-          - os: macos
+          - os: macos-13
             ruby: '2.5'
           - os: macos
             ruby: '3.2'
           - os: windows
             ruby: '2.5'
           - os: windows
             ruby: '3.2'
+            
     runs-on: ${{ matrix.os }}-latest
     steps:
       - uses: actions/checkout@v4