channeldb: Fix migration 20 incorrect outpoint serialization #542

Job	Run time
Build (1.20)	2m 30s
Build (1.21)	2m 37s
Test (1.21, unit-race)	17m 2s
Test (1.21, itest-parallel-run)	19m 42s
Test (1.21, itest-parallel-run walletimpl=embeddedwallet_dcrw)	19m 42s
Test (1.21, itest-parallel-run walletimpl=embeddedwallet_dcrw backend=spv)	19m 57s
Test (1.21, itest-parallel-run walletimpl=remotewallet)	22m 8s
Test (1.21, itest-parallel-run walletimpl=remotewallet backend=spv)	22m 15s
	2h 5m 53s

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