Stateless mode: make it a supported feature

Description

As a follow-up to #49469 where this feature became preview in 26.7, this now focuses on the work necessary to make it fully supported.

We'll collect all issues with feedback as sub-tasks, and eventually then make it supported.

Value Proposition

Provide a multi-cluster setup for users.

Goals

• Make this feature supported and hardened so it can work in production environments.
• Decide if we want to deprecate multi-cluster v1

Non-Goals

• Multi-region setups

Discussion

https://github.com/keycloak/keycloak/discussions/50640

Notes

Optional tasks:

• Use a deployment in the Keycloak Operator