Change pallet referenda TracksInfo::tracks to return an iterator #11012

Job	Run time
check-dependency-rules	12s
check-workspace	32s
check-fail-ci	12s
check-markdown	17s
preflight / preflight	49s
preflight / ci-versions	56s
check-rust-feature-propagation	2m 4s
fmt	1m 2s
check-toml-format	1m 2s
check-umbrella	2m 11s
test-rust-features	2m 1s
All quick checks passed	1s
	11m 19s

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