CI: Add clang 21&22 to Ubuntu CLang build matrix

[petrbel]

Current state: The Ubuntu CI build matrix (ci_test_compilers_clang) pins many Clang versions explicitly, but stops at Clang 20 before jumping to latest. Clang 21 was therefore only covered incidentally, via the latest tag.

Problem: silkeh/clang:latest has moved to Clang 22 and we've silently lost coverage of Clang 21.

Proposed change: Add an explicit 21 and 22 entries to the Ubuntu Clang build matrix. Both silkeh/clang:21 and silkeh/clang:22 images already exist.

Discussion: #5345 , Contribution Guidelines: Wanted

Scope: CI-only. No source, functionality, tests, docs, or coverage affected.

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In case this is mandatory, I comply with Contribution Guidelines, namely Developer Certificate of Origin (DCO) version 1.1

[petrbel]

sorry about the push-force
I didn't know about mandatory sign off

[nlohmann]

Once the CI is green, please also update the docs/mkdocs/docs/community/quality_assurance.md page with the new Clang version that shipped this morning for latest.

[petrbel]

Hi @nlohmann, thanks for review. I totally forgot that.

21.1.8 is already there but I added also 22.1.8 (not a typo, same minor/patch versions) because I happened to get my MR merged today and now there is also silkeh/clang:22. So now CI also builds both 21 and 22.

Hope it's fine now, please let me know if you want something changed.

[nlohmann]

Clang was updated and the latest develop branch fixes a CI issue. Please rebase to the latest develop branch.

[petrbel]

Don't know why Windows / clang-cl-12 (Win32) failed. Maybe some network error?

  6/105 Test   #1: download_test_data .............................***Failed    2.67 sec

Unfortunatelly I'm not allowed to rerun the CI.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!