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.

.github/workflows/ci.yml

@@ -284,8 +284,6 @@ jobs:
           - clang: 5
           - clang: 5
             std: 14
             std: 14
           - clang: 10
           - clang: 10
-            std: 20
-          - clang: 10
             std: 17
             std: 17
           - clang: 11
           - clang: 11
             std: 20
             std: 20