Implement new GA release process

[frouioui]

This issue is derived from vitessio/vitess#15586.

Here is the outline of what needs to be done:

• Only do code freeze during RC-1 and patch releases
• Create two branches when doing RC/GA releases
• Change the release PRs (when in RC) to not unfreeze the release branch until GA
• Add requirements for release Add a list of prerequisites before even doing a release #65