Implement new release process

[frouioui]

This PR is a follow up of the discussion made on https://github.com/vitessio/vitess/issues/15586. It implements the new release process, the following things are changed:

• For a RC/GA release, we only freeze the branch once: during RC-1
• We only unfreeze the branch in the release PR when doing a patch release or a GA release
• We create two release branches (release-20.0 and release-20.0-rc) when doing RC-1
• When doing RC-1 we now duplicate branch protection rules to both new branches
• When doing RC-1 we now create two Backport to labels, one for each branches
• Copy release notes on both branches: main and the real release branch
• Do back to dev mode on both branch
• Enhance the UI of the release issue
• Add a general pre-requisite guides on what's needed to do a release

To test all these changes I have performed 3 RC releases followed by a GA release:

• https://github.com/frouioui/vitess/issues/338
• https://github.com/frouioui/vitess/issues/334
• https://github.com/frouioui/vitess/issues/329
• https://github.com/frouioui/vitess/issues/319

Fixes https://github.com/vitessio/vitess-releaser/issues/88 Fixes https://github.com/vitessio/vitess-releaser/issues/65