Add clang-14 to CI

[j-stephan]

This is a successor PR to #1713.

• Adds clang-14 to CI.
• Adds clang-14 as CUDA compiler to GitLab CI (CUDA versions 9.2 - 11.5).
• ~~Adds clang-14 as host compiler for nvcc to GitLab CI (CUDA 11.6).~~ Edit: Reverted, not supported by CUDA at the moment.
• ~~Enabled OpenMP checks for clang-14 as CUDA compiler (see #639).~~ Edit: Reverted, requires separate PR.
• README.md updated to reflect these changes.
• README.md erroneously stated that clang-13 wouldn't support OpenMP 5 (we test this in the CI already). Fixed.

Fixes #1670. ~~Related to #639; if this goes through a fix is trivial.~~

[j-stephan]

Just FYI, this is on hold until August. I'm currently occupied with preparing a publication.