Rollup merge of rust-lang#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#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#65225

Author: Zalathar

library/alloc/src/collections/btree/map/entry.rs

@@ -271,7 +271,6 @@ impl<'a, K: Ord, V, A: Allocator + Clone> Entry<'a, K, V, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(btree_entry_insert)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map: BTreeMap<&str, String> = BTreeMap::new();
@@ -280,7 +279,7 @@ impl<'a, K: Ord, V, A: Allocator + Clone> Entry<'a, K, V, A> {
     /// assert_eq!(entry.key(), &"poneyland");
     /// ```
     #[inline]
-    #[unstable(feature = "btree_entry_insert", issue = "65225")]
+    #[stable(feature = "btree_entry_insert", since = "CURRENT_RUSTC_VERSION")]
     pub fn insert_entry(self, value: V) -> OccupiedEntry<'a, K, V, A> {
         match self {
             Occupied(mut entry) => {
@@ -379,7 +378,6 @@ impl<'a, K: Ord, V, A: Allocator + Clone> VacantEntry<'a, K, V, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(btree_entry_insert)]
     /// use std::collections::BTreeMap;
     /// use std::collections::btree_map::Entry;
     ///
@@ -391,7 +389,7 @@ impl<'a, K: Ord, V, A: Allocator + Clone> VacantEntry<'a, K, V, A> {
     /// }
     /// assert_eq!(map["poneyland"], 37);
     /// ```
-    #[unstable(feature = "btree_entry_insert", issue = "65225")]
+    #[stable(feature = "btree_entry_insert", since = "CURRENT_RUSTC_VERSION")]
     pub fn insert_entry(mut self, value: V) -> OccupiedEntry<'a, K, V, A> {
         let handle = match self.handle {
             None => {