Add Clang 21 to ubuntu.yml CI (ci_test_compilers_clang)

[petrbel]

I was looking at the explicit Clang version matrix in the Ubuntu workflow and noticed it currently ends at 20 (before jumping to latest). I'd like to suggest adding 21 explicitly, since the Docker image already exists. I've tested the build in that image and it works.

I know Clang 21 is effectively covered today by the latest tag but once that tag moves we'll silently lose coverage of 21. I suspect that's coming soon: Clang 22 has been out for a while now (currently 22.1.8). I'd suggest keeping latest as well, though that's worth discussing.

Btw docs/mkdocs/docs/community/quality_assurance.md already mentions the support of 21.1.8.

Happy to open a PR and test it if you think it's worthwhile, given how small the diff would be :)

[nlohmann]

We pull silkeh/clang:latest which reports 21.1.8. Not sure how to get 22.

[nlohmann]

That said - PRs more than welcome!

[petrbel]

I added clang 21 in this PR: #5347

Clang 22 isn't built for silkeh/clang as of today, however, clang 22 seems to be well running on all reported Debian versions. I create a merge request adding clang 22 image. Hopefully it gets merged soon. If so, I'll add it to nlohmann/json CI then.