GH: remove clang21 from msan builds

Not used. clang22 stable enough.

Author: grooverdan

.github/workflows/build-debian.msan-based.yml

@@ -40,12 +40,6 @@ jobs:
             tag: debian12-msan-clang-20
             clang_version: 20
             nogalera: false
-          - image: debian:13
-            platforms: linux/amd64
-            branch: 11.4
-            tag: debian13-msan-clang-21
-            clang_version: 21
-            nogalera: false
           - image: debian:13
             platforms: linux/amd64
             branch: 11.8