Draft: Replace server awareness implementation

What does this MR do and why?

We previously implemented a stateful server-side Awareness solution for the Issue sidebar. Due to UX concerns, this never shipped, and in the meantime, the CRDT project (y-rb) has reached a level of maturity where we can actually start to use it. This MR aims to remove the server-side Awareness implementation and replace it with an Awareness CRDT.

This is in conjunction with other MRs. The new approach includes page and field-level awareness information as described in detail in !101757 (closed).

Screenshots or screen recordings

No user facing changes and setup (unit tests only).

How to set up and validate locally

No user facing changes and setup (unit tests only).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.