Run the Antora workflow on macOS 15, in GitHub Actions

Author: gennaroprota

.github/workflows/ci.yml

@@ -51,7 +51,7 @@ jobs:
         include:
           - { name: Windows, os: windows-latest }
           - { name: Ubuntu, os: ubuntu-latest }
-          - { name: macOS, os: macos-latest }
+          - { name: macOS, os: macos-15 }
     runs-on: ${{ matrix.os }}
     defaults:
       run: