Remove GCC 7 from CI because the macos CI machines cannot install it anymore

.github/workflows/main.yaml

@@ -13,12 +13,6 @@ jobs:
           - ubuntu-18.04
           - ubuntu-20.04
         compiler:
-          - cxx: g++-7
-            cc: gcc-7
-            macos_cxx: g++-7
-            macos_cc: gcc-7
-            homebrew_package: gcc@7
-            apt_package: g++-7
           - cxx: g++-8
             cc: gcc-8
             macos_cxx: g++-8
@@ -403,7 +397,7 @@ jobs:
         with:
           gtest_args: ${{matrix.gtest_args}}
           extra_env_vars: ${{matrix.extra_env_vars_for_test}}
-          
+
   windows:
     name: CI (Windows)
     strategy: