Authority cancelling upgrade can lead to accidental resets of the upgrade sequence

[colin-axner]

Summary

If the authority cancels the upgrade, they are not required to specify an error receipt which is proved, yet we use that error receipt when updating the upgrade sequence. The authority might do a valid cancellation and accidentally reset their upgrade sequence to 0.

Maybe ChanUpgradeCancel should return the upgrade sequence to update to:

• channel.UpgradeSequence if authority cancels without proof
• errorreceipt.UpgradeSequence if error receipt is proved

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[colin-axner]

update docs written in #5274 when fixing this issue to specify what the upgrade sequence will be set to

[charleenfei]

nice catch @colin-axner!