CI: drop GAP 4.9 from tests, its GAPDoc is too old

Author: fingolfin

.github/workflows/CI.yml

@@ -28,7 +28,6 @@ jobs:
           - stable-4.12
           - stable-4.11
           - stable-4.10
-          - stable-4.9
 
     steps:
       - uses: actions/checkout@v3