Release planning for 1.0.0

[consideRatio]

Let's track work towards a 1.0.0 release of binderhub and start getting versioned releases out! I know at least me and @GeorgianaElena will be able to help towards this.

Quite important things

• Add RELEASE.md and ensure function of automation for tagged releases #1666
• Update to use z2jh 3.0.0 / jupyterhub 4
  • Update to jupyterhub 3.0.0-beta.1, test k8s 1.27, require k8s 1.23+ #1714

Bonus things

• Allow building the image without needing to launch it #1647
• Relocate binderhub's frontend UI from root folder #1707

[consideRatio]

@manics @minrk @rgaiacs do you have issues/prs you want to get solved/merged before a 1.0.0 would be made?

[rgaiacs]

Not from me.

[manics]

I'd like to get some of #1637 . I'll review whether it potentially involves breaking changes or deprecations- if it doesn't then it can be added in a later release.

The other big thing is to ensure there's no leftover bugs from the change to using Traitlets in the build and related classes
The mybinder GKE switch-off uncovered several issues.

A final consideration is whether there's anything we can deprecate/drop now to make maintenance easier in future?