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.