Revert "CI: don't test against GAP 4.11"

This reverts commit eeebdaa.

It was applied to this repository by accident (my mistake)

Author: fingolfin

.github/workflows/CI.yml

@@ -29,7 +29,7 @@ jobs:
           - stable-4.14
           - stable-4.13
           - stable-4.12
-          #- stable-4.11  # fails due to warnings in loading AtlasRep; seems safe to ignore
+          - stable-4.11
           - stable-4.10
           - stable-4.9