Implement new release process

[frouioui]

This PR is a follow up of the discussion made on vitessio/vitess#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:

• Release of v20.0.0 frouioui/vitess#338
• Release of v20.0.0-RC3 frouioui/vitess#334
• Release of v20.0.0-RC2 frouioui/vitess#329
• Release of v20.0.0-RC1 frouioui/vitess#319

Fixes #88
Fixes #65