Add storage bounds for pallet `staking` #15238

Job	Run time
check-dependency-rules	8s
check-markdown	16s
check-workspace	17s
check-fail-ci	23s
check-readme	3m 6s
preflight / preflight	42s
preflight / ci-versions	1m 24s
fmt	1m 23s
check-rust-feature-propagation	2m 28s
check-umbrella	2m 46s
check-toml-format	1m 18s
test-rust-features	2m 51s
All quick checks passed	1s
	17m 3s

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