CI: stop testing against Julia 1.9 (#1878)

Author: fingolfin

.github/workflows/CI.yml

@@ -27,9 +27,8 @@ jobs:
       matrix:
         julia-version:
           - '1.6'
-          - '1.9'   # to be removed in the near future
           - '1.10'
-          - '1.11-nightly'
+          - '1.11'
           - 'nightly'
         os:
           - ubuntu-latest