Make timeline a getter

[florianduros]

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

When looking for stuck unreads, we noticed sometimes that timeline and this.getLiveTimeline().getEvents() are different.

timeline is a copy of this.getLiveTimeline().getEvents() which is manually updated. To avoid missing copy, timeline becomes a getter of this.getLiveTimeline().getEvents().

Also make timeline deprecated.

After some days of testing it, it seems to get rid of the remaining stuck unreads and make disappear some of the zombie notifications.

Closes element-hq/element-web#26878
Part of element-hq/element-web#24392

Notes: Fix cases of stuck unreads and zombie notifications (comes back after a refresh)

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix cases of stuck unreads and zombie notifications (comes back after a refresh) (#4022). Fixes Huge amounts of log spam around hasUserReadEvent event ID X not found in room Y: this shouldn't happen! element-hq/element-web#26878. Contributed by @florianduros.

[andybalaam]

Looks good, but definitely needs testing before merging.

[ThisIsMissEm]

This fixes my bug with Threads being marked permanently as unread, despite being read (they'd revert to unread after restart). Now I can actually mark all the threads as read (if I don't care about them)