Refactor `pallet-grandpa` to use `v2` benchmarks #7303

Job	Run time
preflight / preflight	43s
preflight / ci-versions	1m 4s
build (polkadot-availability-recovery, availability-recovery-regression-bench)	7m 20s
build (polkadot-availability-distribution, availability-distribution-regression-bench)	5m 5s
build (polkadot-node-core-approval-voting, approval-voting-regression-bench)	22m 35s
build (polkadot-statement-distribution, statement-distribution-regression-bench)	4m 5s
publish-benchmarks	0s
	40m 52s

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