Setting the label `ci:skip-news` will skip the job. This is useful for
maintainers to indicate to contributors that a feature isn't big enough
to warrant a news entry, or for contributors who dislike red CI even if
there's nothing wrong.
Also change label `ci-s390x` to `ci:s390x`; this way it'll be easier to
see that `ci:` are a subcategory of labels that affect CI in some way.
If any commit message in the PR is either of type "feat" or is a
breaking change, then there's a high probability that news.txt should be
updated. Give an error if news.txt hasn't been updated in that case.
This workflow cannot 100% correctly determine if news.txt should be
updated even if the commit messages were exactly correct. The entries in
news.txt is determined by changes between releases, while the commit
messages are based on the master branch. While it is an approximation,
it is still a useful enough one that it's still valuable to have this
job as a reminder even if it gives an error if it shouldn't. In these
cases it is perfectly fine to ignore the failure for this job.
