fix(developer): non-minimal BCP 47 tags in kps should be a hint

Fixes #9266.

For historical reasons there are many non-minimal BCP 47 tags in package
metadata in the repository (over 2500). Furthermore, earlier versions of
kmcomp did not fail the build on package compilation warnings. This
means it is better to reduce WARN_LanguageTagIsNotMinimal to
HINT_LanguageTagIsNotMinimal, so that we can otherwise respect the
'treat warnings as errors' flag on the keyboard projects.

We may be able to upgrade this to a warning again one day in the future.