Move MSC3030 `/timestamp_to_event` to stable endpoint

[MadLittleMods]

MSC3030 was merged which means it's now part of the stable Matrix spec.

Unstable /timestamp_to_event -> stable endpoint

We should update /_matrix/client/unstable/org.matrix.msc3030/rooms/<roomID>/timestamp_to_event?ts=<timestamp>&dir=<direction> to be at /_matrix/client/v1/rooms/<roomID>/timestamp_to_event?ts=<timestamp>&dir=<direction>.

Is v1 correct here? I don't get why we use v3 in some cases, etc

Advertising support

Also in terms of clients knowing whether the stable endpoint is supported now; I assume we have to wait until a Matrix 1.6 spec is cut to advertise support like #14184

But MSC3743 for standardizing the error response of unknown endpoints was also just merged which seems relevant. Clients could also check whether the /timestamp_to_event endpoint gives a 404 M_UNRECOGNIZED response to check for support.

[clokep]

Is v1 correct here? I don't get why we use v3 in some cases, etc

Yes, new endpoints get added as v1.

Also in terms of clients knowing whether the stable endpoint is supported now; I assume we have to wait until a Matrix 1.5 spec is cut to advertise support

This depends on whenever it gets merged into the spec, I think? There's not currently a spec PR for it. (Aka this isn't up to us!)

[MadLittleMods]

For reference: This landed as stable in Synapse 1.73.0 (search for #14471)