Fix invalid state transitions in pallet-bounties (#11630)

* Pallet-bounty: disallow invalid state transitions

* Fix tests: funding only at even block numbers

* Fix benchmarks: bounties need to be funded

* fix on_initialize

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/bounties/src/lib.rs

@@ -390,7 +390,7 @@ pub mod pallet {
 			Bounties::<T, I>::try_mutate_exists(bounty_id, |maybe_bounty| -> DispatchResult {
 				let mut bounty = maybe_bounty.as_mut().ok_or(Error::<T, I>::InvalidIndex)?;
 				match bounty.status {
-					BountyStatus::Proposed | BountyStatus::Approved | BountyStatus::Funded => {},
+					BountyStatus::Funded => {},
 					_ => return Err(Error::<T, I>::UnexpectedStatus.into()),
 				};