Use Qt 6 for macOS workflow

- current macOS does no longer support older Qt versions
- the GH runners for older macos versions are deprecated
  and will be removed in a few months

Author: mrbean-bremen

.github/workflows/build.yml

@@ -207,12 +207,12 @@ jobs:
       matrix:
        macos-version:  ['latest']
        python-version: ['3.9']
-       qt-version: ['5.9.*']
+       qt-version: ['6.11.*']
        configuration: ['release','debug']
        include:
          - macos-version: 'latest'
            python-version: '3.11'
-           qt-version: '5.12.*'
+           qt-version: '6.11.*'
            configuration: 'debug'
     runs-on: macos-${{ matrix.macos-version }}
     steps: