Add more sophisticated release note parsing to the epic and issue ref linter

In order for us to properly limit the error messages thrown in the epic and issue ref linter, we need more sophisticated release note parsing. There can be multiple release notes within a single commit, so we need some way to distinguish commits that only have bug fix RNs to commits that have a mixture of bug fix RNs and non-bug fix RNs.

Additionally, the RegEx used to identify bug fix RNs needs to be updated to allow for `bugfix` and `bug fix`.

Jira issue: CRDB-24017

Start by locating the epic and issue ref linter and its release-note parsing logic. Inspect how multiple release notes in one commit are classified and how bug-fix release notes are matched; done means mixed commits are distinguished correctly and both `bugfix` and `bug fix` are recognized.

Written by the indexing model from the issue text.