Rollup merge of #110234 - marc0246:btree-insert-after-fix, r=cuviper

Fix btree `CursorMut::insert_after` check

Fixes a check inside `BTreeMap`'s `CursorMut::insert_after`, where it would peek the previous element to check whether the inserted key is below the next one, instead of peeking the next element.

Author: matthiaskrgr