Add CI builds using clang-19 and clang-20

[MarcelloPerathoner]

Issue

The CI build does not test modern compilers.

PR

This PR:

• adds clang-19 and clang-20 to the CI build
• puts the list of runners in a more logical order

Note: clang-21 and gcc-15 are still missing because the ubuntu-24.04 runner cannot install them from its pre-configured repositories. We can wait for ubuntu-26.04 in April or install clang-21 from the LLVM repositories.

Tasklist

• CHANGELOG.md entry (How to write a changelog entry)
• update relevant Wiki pages
• add tests (see testing documentation)
• review
• adjust for comments

[TheMarex]

Looks good to me, thank you!