staking: only disable slashed validators and keep them disabled for whole era (#9448)

* session: remove disabled validators threshold logic

* staking: add logic to track offending validators

* staking: disable validators for the whole era

* frame: fix tests

* staking: add tests for disabling validators handling

* staking: fix adding offending validator when already slashed in era

* address review comments

* session, staking: add comments about sorted vecs

Co-authored-by: Andronik Ordian <write@reusable.software>

substrate/frame/babe/src/lib.rs

@@ -926,8 +926,8 @@ impl<T: Config> OneSessionHandler<T::AccountId> for Pallet<T> {
 		Self::enact_epoch_change(bounded_authorities, next_bounded_authorities)
 	}
 
-	fn on_disabled(i: usize) {
-		Self::deposit_consensus(ConsensusLog::OnDisabled(i as u32))
+	fn on_disabled(i: u32) {
+		Self::deposit_consensus(ConsensusLog::OnDisabled(i))
 	}
 }