Update julia versions in CI

Author: lgoettgens

.github/workflows/CI.yml

@@ -31,7 +31,8 @@ jobs:
         julia-version:
           - '1.10'
           - '1.11'
-          - '1.12-nightly'
+          - '1.12'
+          - '1.13-nightly'
           - 'nightly'
         os:
           - ubuntu-latest

.github/workflows/gap.yml

@@ -33,7 +33,7 @@ jobs:
           - 'stable-4.15'
         julia-version:
           - '1'         # latest stable release
-          - '1.12-nightly'
+          - '1.13-nightly'
           - 'nightly'
         os:
           - ubuntu-latest