Rollup merge of #144871 - Kivooeo:btree_entry_insert-stabilize, r=jhpratt

Stabilize `btree_entry_insert` feature

This stabilises `btree_map::VacantEntry::insert_entry` and `btree_map::Entry::insert_entry`, following the FCP in [tracking issue](rust-lang/rust#65225).

New stable API:

```rust
impl<'a, K: Ord, V, A: Allocator + Clone> Entry<'a, K, V, A> {
    pub fn insert_entry(self, value: V) -> OccupiedEntry<'a, K, V, A>;
}

impl<'a, K: Ord, V, A: Allocator + Clone> VacantEntry<'a, K, V, A> {
    pub fn insert_entry(mut self, value: V) -> OccupiedEntry<'a, K, V, A>;
}
```

(FCP ended almost a year ago, so if it's needed for process we could rerun it)

Closes: rust-lang/rust#65225

Author: Zalathar