Response formatting for channel upgrades msg server handlers

[damiannolan]

Summary

Discuss alternatives for formatting response information in channel upgrades msg server handlers.

See thread and thread.

We currently chose to opt for returning (Channel, Upgrade) from WriteUpgradeTryChannel in order to have the response values up to date. However, ideally we would keep WriteX funcs void, with zero return args.

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[chatton]

Discussing with @damiannolan , we can keep the WriteUpgradeTryChannel fn the same, but update WriteUpgradeInitChannel to use the same method signature (return Channel, Upgrade) to stay consistent.

[crodriguezvega]

The signature of WriteUpgradeInitChannel changed already in #5264, therefore the issue can be closed.