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/fuzzer/src/mock.rs

@@ -119,7 +119,7 @@ impl pallet_session::SessionHandler<AccountId> for TestSessionHandler {
 		_: &[(AccountId, Ks)],
 	) {}
 
-	fn on_disabled(_: usize) {}
+	fn on_disabled(_: u32) {}
 }
 
 impl pallet_session::Config for Test {