Add flushing check to WriteUpgradeAckChannel

[colin-axner]

Summary

In WriteUpgradeAckChannel add:

if  !k.hasInflightPackets(ctx, portID, channelID) {
    channel.FlushStatus = types.FLUSHCOMPLETE
}

It would be nice to have some tests as well (maybe some unit tests for the write func)

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[chatton]

looks like @charleenfei got this one over the line thanks!