ci: remove clang 10 C++20 (it broke recently) (#4438)

* Remove clang 10 C++20 (it broke recently), add clang 15 C++20 while we are at it.

* No luck trying clang15: Error response from daemon: manifest for silkeh/clang:15 not found: manifest unknown: manifest unknown. Giving up, this needs to be a separate PR.

Author: rwgk

.github/workflows/ci.yml

@@ -283,8 +283,6 @@ jobs:
         include:
           - clang: 5
             std: 14
-          - clang: 10
-            std: 20
           - clang: 10
             std: 17
           - clang: 11