GHA: Replace GCC 12 by GCC 11 temporarily

There is a Boost bug (boostorg/thread#364) in
1.69 causing a build failure with GCC 12 related to `PTHREAD_STACK_MIN`.
This is fixed in Boost 1.73 but that triggers a warning in GCC12: boostorg/test#357
Hence use GCC 11 and Boost 1.73

.github/workflows/unit-tests.yml

@@ -35,7 +35,7 @@ jobs:
           - { compiler: clang-11, os: ubuntu-20.04, buildType: Debug, cxxStandard: 17 }
           - { compiler: clang-11, os: ubuntu-20.04, buildType: Release, cxxStandard: 14 }
           # Linux Latest GCC
-          - { compiler: gcc-12,   os: ubuntu-22.04, buildType: Debug, cxxStandard: 14, coverage: true }
+          - { compiler: gcc-11,   os: ubuntu-22.04, buildType: Debug, cxxStandard: 14, coverage: true }
           # Linux Latest clang
           - { compiler: clang-16, os: ubuntu-20.04, buildType: Debug, cxxStandard: 14, externalSanitizer: true }