Migrate pallet-child-bounties benchmark to v2 #13799

Job	Run time
preflight / preflight	46s
preflight / ci-versions	1m 0s
test-linux-stable-int	9m 55s
test-linux-stable-runtime-benchmarks	31m 55s
test-linux-stable (1/3, parity-large)	40m 10s
test-linux-stable-no-try-runtime (1/2)	37m 49s
test-linux-stable (1/3, parity-oldlinux)	36m 56s
test-linux-stable-no-try-runtime (2/2)	29m 58s
test-linux-stable (2/3, parity-large)	28m 4s
test-linux-stable (2/3, parity-oldlinux)	25m 24s
test-linux-stable (3/3, parity-large)	23m 21s
test-linux-stable (3/3, parity-oldlinux)	22m 57s
All tests passed	1s
	4h 48m 16s

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