Cleanup Github Actions and fix fprettify

[JamieJQuinn]

This PR fixes a number of issues:

• fprettify will now ignore the entire file src/cuda/exec_dist.f90 to avoid it messing with the CUDA chevrons
• Github Actions no longer reports fprettify failing as a true failure; instead it posts a warning, however there's now no obvious indication that a run has failed.
• Updated various github actions to latest versions and pinned their Python version.
• Removed check for Conventional Commits.

Note: the fprettify chevron bugfix has been submitted so the above hack to just remove the file entirely will be no longer needed once the corresponding PR has been merged and fprettify updated.

[Nanoseb]

Looks good to me. Though I'd also remove mentions of the "conventional commit" thing in the documentation. I think it is mainly here: https://github.com/xcompact3d/x3d2/blob/main/docs/source/developer/contributing_guidelines.rst