We should remove experimental support for MSC3852 as it has now closed #14836

This issue has been migrated from #14836.

MSC3852 has been closed due to the only supporting client implementation (and thus use case) being shuttered. The MSC was implemented in Synapse in matrix-org/synapse#13549.

This endpoint is still in use by the current implementation of the Device Manager in Element clients, so I feel we should support it for a little bit longer until that implementation is rewritten to no longer use this endpoint, and some time further after that still. But we should eventually remove it.

No wider deprecation period is necessary as this is an experimental feature. But it would be nice to coordinate with the Element devs before doing so as not to pull the rug out from under them.