Stop testing with GCC 7 and 8

Support for GCC 7 and 8 can continue on an as-needed basis. The
goal is not to explicitly break support for such compilers. Chances
are they will continue to compile Catch2 for some time into the
future. Rather, it no longer seems a prudent use of resources to
continuously test with these compilers.

.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
@@ -98,8 +92,6 @@ jobs:
         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}} \