ci: replace macos-12 workflows with newer ones

GitHub Actions does not support `macos-12` anymore - let's replace
it with the newer `macos-15` version.

Author: drewdzzz

.github/workflows/testing.yml

@@ -54,12 +54,12 @@ jobs:
           - cxx-standard: 17
             compiler: {c: gcc, cxx: g++}
           # gcc on macos is just an alias for clang
-          - runs-on: macos-12
-            compiler: {c: gcc, cxx: g++}
           - runs-on: macos-13
             compiler: {c: gcc, cxx: g++}
           - runs-on: macos-14
             compiler: {c: gcc, cxx: g++}
+          - runs-on: macos-15
+            compiler: {c: gcc, cxx: g++}
     name: build (${{ matrix.runs-on }}, ${{ matrix.build-type }}, ${{ matrix.compiler.c }}, C++${{ matrix.cxx-standard }})
     with:
       build-only: true