Room version 11 & MSC2176: Only preserve entirety of content instead of whole event

[turt2live]

Description

At

synapse/synapse/events/utils.py

Line 141 in 67f9e52

return event_dict

the entire event, unredacted, is returned in v11 rooms, which is not what MSC2176 calls for.

From the MSC (emphasis mine):

The following are to be added to the list of subkeys of the content property which are preserved:

• m.room.create preserves all content. [...]

Therefore, we should be keeping content unredacted but otherwise removing other fields.

Context

This came up as part of Linearized Matrix interop testing. Synapse still includes origin on outbound events though under v11 they are supposed to be redacted as they serve no meaningful purpose in modern room versions. Because of the return mentioned above, Synapse was not redacting origin off the create event, but was redacting it off all other events. This lead to signature errors when not-Synapse was trying to implement the LM room version (which is v11 plus stuff not relevant here).

[turt2live]

1e77a62 fixes this without tests (and the conversation mentioned in the commit description is resolved here)

[anoadragon453]

Fixed by #15973.