Parachains: Use relay chain slot for velocity measurement #16113

Job	Run time
check-dependency-rules	9s
check-workspace	17s
check-fail-ci	12s
check-markdown	14s
check-readme	3m 2s
preflight / preflight	42s
preflight / ci-versions	55s
check-rust-feature-propagation	2m 1s
fmt	1m 14s
test-rust-features	1m 57s
check-toml-format	54s
check-umbrella	2m 15s
All quick checks passed	1s
	13m 53s

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