[Backport 2.x] Add release notes for 2.12.0 (#1431) (#1438)

* Add release notes for 2.12.0 (#1430)


(cherry picked from commit b15346d)


* Update release-notes/dashboards-observability.release-notes-2.12.0.0.md



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(cherry picked from commit 3afae5c)

Signed-off-by: Ryan Liang <[email protected]>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Signed-off-by: Paul Sebastian <[email protected]>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Paul Sebastian <[email protected]>