Enforce KeepAChangelog in package pipelines

Currently most of our changelogs claim to follow keepachangelog but several definitely have the format wrong.

We should add a CI stage to package CI that validates our changelog is in the right format for MRs. This could use some pre-existing tool (there are some parsers out there that may work) or a custom built solution.

Benefits: standard changelog format for customers, consistent scraping of changelog for package releases, simpler changelog creation at BB level (relates to https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/837).

AC:

• New pipeline change that validates changelog is in keepachangelog format. Stage should fail if changelog is malformed (bonus: pipeline reformats changelog automagically + outputs - some of the parsers out there claim to do this, but not sure if it works well)