apiage: detect APIs that should be stable now

[phlogistonjohn]

The apiage tool should be able to detect when an API is now ready to be marked stable. For example, we could input the current git tag and if the version number is greater than or equal to the "expected stable in version" value for preview apis the tool would produce an error indicating the APIs should be made stable.

This could also be a specific CI job - running this on PRs would not help contributors but a specific job for maintainers could serve as a reminder to fix up the docs and build tags, etc.