Add GCC-11 to workflow

Author: awvwgk

.github/workflows/CI.yml

@@ -21,7 +21,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
-        gcc_v: [7, 8, 9, 10] # Version of GFortran we want to use.
+        gcc_v: [7, 8, 9, 10, 11] # Version of GFortran we want to use.
     env:
       FC: gfortran-${{ matrix.gcc_v }}
       GCC_V: ${{ matrix.gcc_v }}