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/staking/src/pallet/impls.rs

@@ -302,6 +302,13 @@ impl<T: Config> Pallet<T> {
 				Self::start_era(start_session);
 			}
 		}
+
+		// disable all offending validators that have been disabled for the whole era
+		for (index, disabled) in <OffendingValidators<T>>::get() {
+			if disabled {
+				T::SessionInterface::disable_validator(index);
+			}
+		}
 	}
 
 	/// End a session potentially ending an era.
@@ -374,6 +381,9 @@ impl<T: Config> Pallet<T> {
 			// Set ending era reward.
 			<ErasValidatorReward<T>>::insert(&active_era.index, validator_payout);
 			T::RewardRemainder::on_unbalanced(T::Currency::issue(rest));
+
+			// Clear offending validators.
+			<OffendingValidators<T>>::kill();
 		}
 	}