chore(main): release 2.21.0 (#2532)

* chore(main): release 2.21.0

* chore: Update documentation to latest version.

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Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
Co-authored-by: kgala2 <galakp@google.com>