bump qt version to qt6 for macos-14

Author: andrewkern

.github/workflows/tests.yml

@@ -198,10 +198,11 @@ jobs:
           # Ubuntu 24.04 with the most recent Qt6 and GCC.
           - { os: ubuntu-22.04, qt: 6.8.2, gcc: 12 }
           # old macOS with oldest supported Qt5; macos-12 and macos-13 were removed by GitHub,
-          # so we can't CI with Qt 5.9.5 any more, oh well
+          # so we can't CI with Qt 5.x any more (no ARM64 builds available), oh well
           #- {os: macos-12, qt: 5.9.5}
-          # old macOS with last official LTS Qt5
-          - { os: macos-14, qt: 5.15.2 }
+          #- {os: macos-13, qt: 5.15.2}
+          # macOS ARM64 with Qt6
+          - { os: macos-14, qt: 6.7.3 }
           # new macOS with most recent Qt6
           - { os: macos-15, qt: 6.8.2 }
     runs-on: ${{ matrix.os }}