Add Clang 21 CI build (#201)

Author: offa

.github/workflows/ci.yml

@@ -14,8 +14,8 @@ jobs:
         compiler:
           - gcc:15
           - gcc:14
+          - clang:21
           - clang:20
-          - clang:19
     container:
       image: "registry.gitlab.com/offa/docker-images/${{ matrix.compiler }}"
     name: "${{ matrix.compiler }}"