[Chore] v21.0-rc3 release

[serokell-bot]

Description

This PR introduces a new release in tezos-packaging.

Follows #

Changes related to the creation of a release (conditional)

• I updated release number in meta.json.
• If the native release version was updated, I reset the letter_version in model.py.

In case the new Tezos release provides a new protocol and corresponding testnet

• I supported new protocol in protocols.json.
• I supported new protocol and testnet in native packaging.
• I supported new protocol and testnet in brew formulas.
• I added tests for the new protocol and testnet.

Things to do after this release PR is merged

Update brew bottles and repository mirrors

• I merged PR that updates bottles hashes (this PR is supposed to be created by the CI).

Update documentation

• I updated README.md.
• I updated baking doc.