Mark feature flag semantic conventions as release candidate

[dyladan]

Fixes #2249

Changes

Mark feature flag logs semantic conventions and all associated attributes as Release Candidate

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
• ~~[ ] schema-next.yaml updated with changes to existing conventions.~~

[lukas-reining]

Sorry @lmolkova I accidentally disabled auto-merge, but I do not have the permission to enable it again.