Change pallet referenda TracksInfo::tracks to return an iterator #10639

Job	Run time
preflight / preflight	1m 21s
preflight / ci-versions	1m 12s
cargo-check-benches (master)	21m 19s
test-frame-ui	27m 16s
cargo-check-benches (current)	24m 51s
test-full-crypto-feature	2m 36s
cargo-check-all-crate-macos	11m 33s
cargo-check-each-crate (1)	20m 21s
cargo-check-each-crate (2)	19m 0s
test-node-metrics	9m 53s
cargo-check-each-crate (3)	7m 4s
cargo-check-each-crate (4)	18m 1s
cargo-check-each-crate (5)	26m 11s
cargo-check-each-crate (6)	13m 5s
cargo-check-each-crate (7)	27m 43s
test-frame-examples-compile-to-wasm	2m 57s
check-tracing	2m 13s
test-deterministic-wasm	7m 36s
check-metadata-hash	6m 50s
node-bench-regression-guard	12s
All test misc tests passed	1s
	4h 11m 15s

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