Stop testing with GCC 7 and 8

Support for GCC 7 and 8 can continue on an as-needed basis. Chances
are, these compilers will continue to work into the future, but the
value proposition for supporting them is fading. These are already
quite old compilers and if anyone really wants to use them, old
releases of Catch2 are available.

Author: ChrisThrasher

.github/workflows/linux-simple-builds.yml

@@ -10,8 +10,6 @@ jobs:
       fail-fast: false
       matrix:
         cxx:
-          - g++-7
-          - g++-8
           - g++-9
           - g++-10
           - g++-11
@@ -28,10 +26,6 @@ jobs:
         build_type: [Debug, Release]
         std: [14]
         include:
-          - cxx: g++-7
-            other_pkgs: g++-7
-          - cxx: g++-8
-            other_pkgs: g++-8
           - cxx: g++-9
             other_pkgs: g++-9
           - cxx: g++-10
@@ -87,19 +81,12 @@ jobs:
     steps:
     - uses: actions/checkout@v4
 
-    - name: Add repositories for older compilers
-      run: |
-        sudo apt-add-repository 'deb http://azure.archive.ubuntu.com/ubuntu/ focal main'
-        sudo apt-add-repository 'deb http://azure.archive.ubuntu.com/ubuntu/ focal universe'
-
     - name: Prepare environment
       run: |
         sudo apt-get update
         sudo apt-get install -y ninja-build ${{matrix.other_pkgs}}
 
     - name: Configure
-      # Note: $GITHUB_WORKSPACE is distinct from ${{runner.workspace}}.
-      #       This is important
       run: |
         cmake --preset basic-tests -GNinja \
               -DCMAKE_BUILD_TYPE=${{matrix.build_type}} \