[Backport 2.12] Add Release Notes for v2.12.0

[opensearch-trigger-bot[bot]]

Backport aabe211a7940b3dd11202a5bebec2014e4b13502 from #628

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (273a39b) 68.70% compared to head (43dd610) 68.54%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 2.12 #631 +/- ## ============================================ - Coverage 68.70% 68.54% -0.17% + Complexity 365 364 -1 ============================================ Files 43 43 Lines 2473 2473 Branches 172 172 ============================================ - Hits 1699 1695 -4 - Misses 671 674 +3 - Partials 103 104 +1 ```

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