Description:

Since MSC3912 has been implemented by #14260, we introduced 2 changes into the MSC for which we need to update this implementation :

• the new property with_relations has been renamed with_rel_types (to prevent us from introducing a new term ("relations") to refer to event relationships). Only the stable name is changed, we keep the existing unstable one.
• we introduced a catch-all "*" value, which if found in the list means "any relation type"

There is one stuff missing:
The first Synapse PR (#14260) says:

This implementation is lacking a way to keep watch for incoming events that match the redaction criteria after the redaction request finishes, that'll come in a separate PR.

We have to decide if the backend team can work on this border use case by provided an additional PR. If not, we should remove this part from the MSC (details)