Try macos-15, and gcc-14 on macos runners

Because GitHub is about to deprecate macos-12, and macos-15 is
about to leave beta.

See actions/runner-images#10721

.github/workflows/build.yaml

@@ -153,11 +153,11 @@ jobs:
         - 6.6.3
         - 6.7.3
         - 6.8.0
-        os: [ 'macos-12', 'macos-13', 'macos-14' ]
+        os: [ 'macos-13', 'macos-14', 'macos-15' ]
         env:
         - { cc: clang, cxx: clang++, coverage: false }
-        - { cc: gcc-12, cxx: g++-12, coverage: false }
-        - { cc: gcc-12, cxx: g++-12, coverage: true }
+        - { cc: gcc-14, cxx: g++-14, coverage: false }
+        - { cc: gcc-14, cxx: g++-14, coverage: true }
         exclude:
           # Exclude Qt 5.x with GCC on macOS 13 (Qt5 only officially supports up to macOS 12).
           - { qt: '5.9.9',   os: 'macos-13', env: { cc: gcc-12 } }