Remove Github build on Ubuntu 20.04

This is a scheduled Ubuntu 20.04 retirement. Ubuntu 20.04 LTS runner will be removed on 2025-04-15. For more details, see actions/runner-images#11101

Author: SRombauts

.github/workflows/cmake.yml

@@ -42,14 +42,6 @@ jobs:
             cc: "gcc", cxx: "g++",
             extra_path: "",
           }
-        - {
-            name: "Ubuntu 20.04 GCC",
-            os: ubuntu-20.04,
-            generator: "Unix Makefiles",
-            build_type: "Debug",
-            cc: "gcc", cxx: "g++",
-            extra_path: "",
-          }
         - {
             name: "macOS Latest Clang",
             os: macos-latest,