Fix Github Actions build failing on "Ubuntu 16.04 GCC" for 3 months

Author: SRombauts

.github/workflows/subdir_example.yml

@@ -20,11 +20,6 @@ jobs:
             os: ubuntu-latest,
             build_type: "Debug", cc: "gcc", cxx: "g++"
           }
-        - {
-            name: "Ubuntu 16.04 GCC",
-            os: ubuntu-16.04,
-            build_type: "Debug", cc: "gcc", cxx: "g++"
-          }
         - {
             name: "macOS Latest Clang",
             os: macos-latest,