HDDS-16104. Add Ozone 2.1.2 release notes and download page

[jojochuang]

Summary

• Add release notes for Apache Ozone 2.1.2
• Update download page and versions.json
• Add versioned docs for 2.1.2

Tracking

Release tracker: HDDS-16102 — Release version 2.1.2

Sub-task: HDDS-16104 — Add Ozone 2.1.2 download & release note

Test plan

• pnpm build locally
• CI green on this PR

[adoroszlai]

• 2.1.1 should have been removed (for patch release: replace previous version on same minor release line)
• Ideally create separate PR for versioned docs, because that adds lots of generated files, which makes review of the core change more difficult if mixed with download page / release notes.

[jojochuang]

Thanks for the review, @adoroszlai.

Download page: Agreed — for a patch on the same minor line we should replace the previous patch, not add alongside. That is already corrected on master in a8e259e (remove 2.1.1 from the downloads table).

versions.json: I missed the same replacement in #543 — it still lists 2.1.1 alongside 2.1.2. I will open a small follow-up PR to drop 2.1.1 from versions.json, matching what we did for 2.2.0 → 2.2.1 in #539.

Split PRs: Good point on keeping versioned docs in a separate PR from download / release notes. I will do that for future releases (metadata PR first, then the versioned_docs snapshot).

https://issues.apache.org/jira/browse/HDDS-16104