Require authority to pass proof if self channel state is in FLUSHCOMPLETE for upgrade cancellation

[colin-axner]

As per code comment:

// if the msgSender is authorized to make and cancel upgrades AND the current channel has not already reached FLUSHCOMPLETE
// then we can restore immediately without any additional checks
// otherwise, we can only cancel if the counterparty wrote an error receipt during the upgrade handshake

If the channel state is in FLUSHCOMPLETE, regardless of whether the authority called this function, we must prove that the counterparty wrote an error receipt for a valid upgrade sequence. Otherwise the authority could cancel an upgrade while the counterparty simultaneously is provided historical proof to upgrade to open. In this case, the authority would be harming itself, but no harm in ensuring this isn't possible

Originally posted by @colin-axner in #5262 (comment)