Remove support for unstable private read receipts (#13653)

changelog.d/13653.removal

synapse/api/constants.py

synapse/config/experimental.py

synapse/handlers/receipts.py

synapse/replication/tcp/client.py

synapse/rest/client/notifications.py

synapse/rest/client/read_marker.py

synapse/rest/client/receipts.py

synapse/rest/client/versions.py

synapse/storage/databases/main/event_push_actions.py

tests/handlers/test_receipts.py

tests/rest/client/test_sync.py

tests/storage/test_receipts.py