collator-protocol: Handle unknown validator heads #7325

Job	Run time
set-image	10s
check-runtime-migration (westend)	7m 4s
check-runtime-migration (rococo)	4m 51s
check-runtime-migration (asset-hub-westend)	4m 38s
check-runtime-migration (asset-hub-rococo)	6m 2s
check-runtime-migration (bridge-hub-westend)	4m 7s
check-runtime-migration (bridge-hub-rococo)	4m 11s
check-runtime-migration (contracts-rococo)	4m 2s
check-runtime-migration (collectives-westend)	4m 1s
check-runtime-migration (coretime-rococo)	4m 9s
All runtime migrations passed	1s
	43m 16s

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