Add storage bounds for pallet `staking` #15390

Job	Run time
preflight / preflight	43s
preflight / ci-versions	1m 20s
check-runtime-migration (westend)	5m 43s
check-runtime-migration (asset-hub-westend)	5m 42s
check-runtime-migration (bridge-hub-westend)	5m 7s
check-runtime-migration (collectives-westend)	5m 9s
check-runtime-migration (coretime-westend)	5m 15s
All runtime migrations passed	1s
	29m 0s

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