Read receipts on element web are broken

[erikjohnston]

Broke by #12168. Element web sends a hidden read receipt type, which Synapse doesn't understand and so 400s:

synapse/synapse/rest/client/read_marker.py

Lines 57 to 63 in 0ce2201

	if set(body.keys()) > valid_receipt_types:
	raise SynapseError(
	400,
	"Receipt type must be 'm.read', 'org.matrix.msc2285.read.private' or 'm.fully_read'"
	if self.config.experimental.msc2285_enabled
	else "Receipt type must be 'm.read' or 'm.fully_read'",
	)

This is a problem as it stops the other receipt types from being persisted.

We shouldn't be denying unknown receipt types.