Github Action looking for Changelog notes in a section for a previously released version.

[SpicyLemon]

Summary

Create a github action that creates comments/annotations in the changelog when a line is being added to a released section.

Problem Definition

When the changelog in main is updated after a release, it's easy for existing PRs to accidentally add entries to it rather than in the now empty ## Unreleased section further up. It'd also be nice to have a couple automated checks on the changelog changes.

Proposal

Create a github action that analyzes the changes being made to the CHANGELOG by a PR.

1. Add annotations to changes being made to changes outside the ## Unreleased section.
2. If a change is being made outside the ## Unreleased section, make a comment on the first such change section pointing it out.
3. If none of the lines being added contain a link, make a comment pointing that out.
4. If there isn't a changelog entry, make a comment pointing that out.

Notes:

• The action shouldn't make comments on draft PRs.
• We don't want this action spamming a PR with comments every time it's run. Care should be taken to not duplicate comments.
• Ideally, 2 and 3 would only analyze the commits added since the previous push (as opposed to analyzing the PR as a whole).
• For 4, It's not uncommon for us to create a PR before adding a changelog entry so we know the PR number first. So it'd be nice if this check/comment were on a delay.
• If the action created a comment about 4, and an entry is then added, it should then resolve that comment.
• I'm not even sure it's possible for a github action to add a comment. We might need to utilize the same credentials being used in the proto-registry.yml workflow to create a PR.

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[SpicyLemon]

Now that we've switched up how we do the changelog (#2112), this isn't needed; PRs should not be adding things directly to CHANGELOG.md anymore (other than ones marking releases).