Synapse calls `/r0` for media over federation

[turt2live]

synapse/synapse/media/media_repository.py

Line 427 in 4fc8875

("/_matrix/media/r0/download", server_name, media_id)

It should be trying the /v3 endpoint until something like MSC1902 or similar lands. Arguably there should also be a fallback mechanism to try /r0 when /v3 fails, per M_UNRECOGNIZED in the standard error codes.

[clokep]

This is part of #11474, which also talks about a few other spots.