CI Update mac versions for build test

Author: jarzec

.github/workflows/build-cppfront.yaml

@@ -35,9 +35,12 @@ jobs:
           - compiler: clang++-14
             cxx-std: 'c++2b'
         include:
-          - runs-on: macos-latest
+          - runs-on: macos-15
             compiler: clang++
             cxx-std: 'c++20'
+          - runs-on: macos-15
+            compiler: clang++
+            cxx-std: 'c++2b'
           - runs-on: ubuntu-22.04
             compiler: clang++-15
             cxx-std: 'c++20'