ci: replace macos-11 workflows with newer ones

GitHub Actions does not support `macos-11` anymore - let's replace
it with the newer versions.

Author: drewdzzz

.github/workflows/testing.yml

@@ -16,8 +16,9 @@ jobs:
         runs-on:
           - ubuntu-20.04
           - ubuntu-22.04
-          - macos-11
           - macos-12
+          - macos-13
+          - macos-14
         build-type:
           - Debug
           - RelWithDebInfo
@@ -36,8 +37,9 @@ jobs:
         runs-on:
           - ubuntu-20.04
           - ubuntu-22.04
-          - macos-11
           - macos-12
+          - macos-13
+          - macos-14
         compiler:
           - {c: gcc, cxx: g++}
           - {c: clang, cxx: clang++}
@@ -52,10 +54,12 @@ jobs:
           - cxx-standard: 17
             compiler: {c: gcc, cxx: g++}
           # gcc on macos is just an alias for clang
-          - runs-on: macos-11
-            compiler: {c: gcc, cxx: g++}
           - 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++}
     name: build (${{ matrix.runs-on }}, ${{ matrix.build-type }}, ${{ matrix.compiler.c }}, C++${{ matrix.cxx-standard }})
     with:
       build-only: true