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/support/src/traits/validation.rs

@@ -109,7 +109,7 @@ pub trait OneSessionHandler<ValidatorId>: BoundToRuntimeAppPublic {
 	fn on_before_session_ending() {}
 
 	/// A validator got disabled. Act accordingly until a new session begins.
-	fn on_disabled(_validator_index: usize);
+	fn on_disabled(_validator_index: u32);
 }
 
 /// Something that can estimate at which block the next session rotation will happen (i.e. a new