Add migration to clear unapproved proposals from treasury pallet #2406

Job	Run time
preflight / preflight	34s
preflight / ci-versions	1m 19s
test-frame-ui	25m 44s
cargo-check-benches (master)	25m 36s
cargo-check-all-crate-macos	10m 13s
cargo-check-each-crate (1)	21m 41s
cargo-check-benches (current)	27m 25s
cargo-check-each-crate (2)	19m 12s
test-full-crypto-feature	2m 37s
cargo-check-each-crate (3)	5m 10s
test-node-metrics	11m 45s
cargo-check-each-crate (4)	18m 32s
cargo-check-each-crate (5)	26m 37s
cargo-check-each-crate (6)	13m 33s
cargo-check-each-crate (7)	29m 39s
test-frame-examples-compile-to-wasm	2m 57s
test-deterministic-wasm	7m 30s
check-tracing	2m 14s
check-metadata-hash	6m 34s
node-bench-regression-guard	12s
All test misc tests passed	1s
	4h 19m 5s

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