[release-notes] Update automation to use new markdown format

[brianseeders]

The release notes for 9.0 are in an entirely new structure compared to 8.x and prior. Not only are the release notes now in markdown, but all release notes now live in main, instead of on the branch for the given version.

So, when we generate the release notes files, we're now generating release notes for all versions at the same time, instead of just one. To make this possible, I'm introducing ChangelogBundles, which are just yaml files that contain all of the changelogs for a given version, bundled up into one file. That way, we can re-generate all release notes for all versions at any time.

My two main goals, outside of supporting the new format/structure were:

• Keep the changelog process the same for developers working on ES
• Make correcting release notes simpler (it's so complicated right now that basically only ES Delivery folks can do it)

Here's what the process now looks like (it's automated, this is just for reference):

# After a BC is cut:
./gradlew bundleChangelogs --branch <branch> --bc-ref <bc_sha>
./gradlew generateReleaseNotes
# commit the changes

# After the release is finalized/released:
./gradlew bundleChangelogs --branch <branch> --bc-ref <bc_sha> --finalize
./gradlew generateReleaseNotes
# commit the changes

# That's it!

Adding missing changelogs or correcting existing ones after a BC is cut is indeed much simpler:

• If the release and the release notes have not been finalized yet:
  • Add or update the docs/changelog/<PR>.yaml file
  • Commit / PR / Backport
• If the release and notes have been finalized
  • Add or update an entry in the final docs/release-notes/changelog-bundles/<version>.yml file`
  • Run ./gradlew generateReleaseNotes
  • Commit / PR (only needed on main)

I have already been using this branch to generate the release notes for 9.0+.

Once this is merged, I will also update the release automation to use it.

[leemthompo]

FYI this PR to update headings might be relevant to your templates: #125548

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[leemthompo]

Output looks good based on 9.0.2 updates. Thanks for all your work on this @brianseeders !