Enable the `.org.matrix.msc4028.encrypted_event` push rule by default (#17826)

Clients will still only see this rule if the corresponding experimental
feature, `msc4028_push_encrypted_events`, is also enabled.

This aligns the implementation with MSC4028, specifically [this
section](https://github.com/matrix-org/matrix-spec-proposals/blob/giomfo/push_encrypted_events/proposals/4028-push-all-encrypted-events-except-for-muted-rooms.md#unstable-prefix).
See https://github.com/element-hq/synapse/issues/16846 for context.