CI: revert release workflow to ubuntu-latest

With ubuntu-slim, docker containers can't be used, but
update-gh-pages uses an action which needs one.

Author: fingolfin

.github/workflows/release.yml

@@ -19,7 +19,7 @@ permissions: write-all
 jobs:
   release:
     name: "Release the GAP package"
-    runs-on: ubuntu-slim
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v6