pallet-offences: Switch to partition_point (#9049)

This changes the code to use `partition_point` instead of
`binary_search_by_key`, because this was very likely the problematic
pallet 2 weeks ago on polkadot.

substrate/frame/offences/src/mock.rs

@@ -170,3 +170,8 @@ impl<T: Clone> offence::Offence<T> for Offence<T> {
 		Perbill::from_percent(5 + offenders_count * 100 / validator_set_count)
 	}
 }
+
+/// Create the report id for the given `offender` and `time_slot` combination.
+pub fn report_id(time_slot: u128, offender: u64) -> H256 {
+	Offences::report_id::<Offence<u64>>(&time_slot, &offender)
+}