grandpa: Ensure `WarpProof` stays in its limits #16913

Job	Run time
preflight / preflight	41s
preflight / ci-versions	1m 13s
test-linux-stable-int	9m 16s
test-linux-stable-runtime-benchmarks	28m 41s
test-linux-stable-no-try-runtime (1/2)	29m 28s
test-linux-stable (1/3, parity-large)	30m 44s
test-linux-stable-no-try-runtime (2/2)	27m 30s
test-linux-stable (1/3, parity-oldlinux)	30m 15s
test-linux-stable (2/3, parity-large)	26m 31s
test-linux-stable (2/3, parity-oldlinux)	20m 44s
test-linux-stable (3/3, parity-large)	26m 42s
test-linux-stable (3/3, parity-oldlinux)	19m 57s
All tests passed	1s
	4h 11m 43s

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