Synapse spams servers with requests to /_matrix/federation/v1/get_groups_publicised

When the server responds with a 404 to this endpoint, synapse infinitely loops on hitting it again.

Be advised that this can be used as part of a denial of service attack.

FTR the endpoint is not satisfied with a 200 OK, an empty {} or even {"users": {}}

It doesn't stop until you repeat back the users it asked for in the request with empty arrays for each one.

I'm also seeing this on mxhsd

#12563 and related PRs removed the groups feature.
The endpoint in question no longer exists and is no longer requested.

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