Change pallet referenda TracksInfo::tracks to return an iterator #18733

Job	Run time
check-dependency-rules	8s
check-fail-ci	12s
check-markdown	14s
check-workspace	17s
check-readme	3m 56s
preflight / preflight	1m 23s
preflight / ci-versions	1m 12s
fmt	1m 22s
check-rust-feature-propagation	2m 19s
check-toml-format	1m 15s
check-umbrella	2m 15s
test-rust-features	2m 6s
All quick checks passed	1s
	16m 40s

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