Stop sending reply and edit fallbacks (MSC2781)

[aaronraimist]

Fixes element-hq/element-web#20334
Fixes element-hq/element-web#7874
Fixes element-hq/element-web#7014

Implementation of MSC2781 matrix-org/matrix-spec-proposals#2781

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This change is marked as an internal change (Task), so will not be included in the changelog.

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[turt2live]

While the code looks fine, there's ecosystem compatibility to consider on this, including with bots, bridges, other clients, etc.

This PR serves functionally fine for an implementation of the removal, but for the spec side we'd need reassurance that bridges and bots won't break (clients are easier to fix).

[HarHarLinks]

while I agree the ecosystem needs to adapt, I think it's much easier to support events without than with fallbacks also for bots just as it is for client( sdk)s. What's the plan to push towards it? Labs flag for this and test around how it goes? Announce to everyone to implement this MSC before it gets merged while retaining compatibility with fallbacks with a deadline?

[turt2live]

See also: MSC2781

[turt2live]

This PR will probably take a while to merge, as it would require us to have suitable support on mobile (android at least doesn't currently render replies properly). The MSC also appears to be stalled, which isn't good :(

For now I think we can keep this PR open because it's not that old, though if the Android/mobile support or MSC take too long then we may revisit this at a later date.

Code-wise this looks fine. Just marking requested changes for the mobile support (and MSC, though to a lesser degree).

[weeman1337]

Conversion to a draft, as we are still waiting for mobile. We should reopen it if there is a chance to move it forward.

(see Oldest non-draft PR from last Web Weekly)