add explicit cleanup_expired_proposals extrinsic

[n13]

Summary

Refactored expired proposal cleanup in the multisig pallet from automatic (implicit) to explicit (opt-in).

Before: propose, approve, and cancel extrinsics automatically iterated over proposals to clean up expired ones, adding unpredictable gas costs.

After: A new cleanup_expired_proposals extrinsic allows wallets to explicitly trigger cleanup when they detect expired proposals.

Changes

• lib.rs: Removed auto_cleanup_expired_proposals calls from propose, approve, cancel. Added new cleanup_expired_proposals extrinsic (call_index 7).
• weights.rs: Updated weight signatures (removed e parameter from existing extrinsics, added cleanup_expired_proposals(e) function).
• benchmarking.rs: Removed expired proposal setup from existing benchmarks, added dedicated cleanup_expired_proposals benchmark.
• tests.rs: Updated tests to explicitly call the new cleanup extrinsic.

Benefits

• Predictable gas costs for core operations
• Wallets control when cleanup happens
• Cleaner separation of concerns

[n13]

that was already done with remove_expired