Release prep v3.21.0

[github-actions[bot]]

Automated release-prep through pdk-templates from commit d0c72ab3d404ed293e24924f5d6b7947ac9e51df. Please verify before merging:

• [x] last nightly run is green
• [x] Changelog is readable and has no unlabeled pull requests
• [x] Ensure the changelog version and metadata version match