Parachains: Use relay chain slot for velocity measurement #17141

Job	Run time
check-dependency-rules	11s
check-fail-ci	10s
check-markdown	17s
check-readme	3m 9s
check-workspace	16s
preflight / preflight	48s
preflight / ci-versions	57s
fmt	1m 9s
check-rust-feature-propagation	2m 0s
check-toml-format	1m 10s
test-rust-features	1m 57s
check-umbrella	2m 21s
All quick checks passed	1s
	14m 26s

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