Auto merge of #86392 - JohnTitor:use-partition-point, r=petrochenkov

Prefer `partition_point` to look up assoc items

Since we now have `partition_point` (instead of `equal_range`), I think it's worth trying to use it instead of manually finding it.
`partition_point` uses `binary_search_by` internally (#85406) and its performance has been improved (#74024), so I guess this will make a performance difference.