Document release process changes with DRA

[seanstory]

Part of https://github.com/elastic/search-team/issues/8047

Updates our release documentation to clarify process and versioning changes that come from DRA assimilation

Checklists

Pre-Review Checklist

• [x] this PR has a meaningful title
• [x] this PR links to all relevant github issues that it fixes or partially addresses
• [x] this PR has a thorough description
• [x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)
• [x] Considered corresponding documentation changes

Related Pull Requests

• RM template changes: https://github.com/elastic/search-team/pull/8161

[seanstory]

I've reached out to the platform ingest productivity team here