[Story] Threads: Unsubscribing from a Thread

### Description

* As a user
* I want to unsubscribe a thread
* So that I am not disturbed by notifications from discussions I’m not interested in.

This story is likely to be covered at the same time as https://github.com/element-hq/element-meta/issues/2766

### Acceptance criteria
- An "Unsubscribe" option is available in the thread menu.
- Unsubscribed threads no longer trigger notifications.

### Dependencies
- https://github.com/element-hq/element-meta/issues/2777

### Out of scope
- Nothing

### Questions
- [ ]

## Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

Start by reviewing the related story in issue 2766 and the dependency in issue 2777, then determine which Android, iOS, and Rust client work is required. The work is complete when the thread menu offers an "Unsubscribe" option and unsubscribed threads no longer trigger notifications.

Written by the indexing model from the issue text.