Rollup merge of rust-lang#60356 - JohnTitor:stabilize-as-mut-ptr, r=Centril

Stabilize str::as_mut_ptr

Closes rust-lang#58215

Author: Centril

src/libcore/str/mod.rs

@@ -2214,7 +2214,7 @@ impl str {
     /// modified in a way that it remains valid UTF-8.
     ///
     /// [`u8`]: primitive.u8.html
-    #[unstable(feature = "str_as_mut_ptr", issue = "58215")]
+    #[stable(feature = "str_as_mut_ptr", since = "1.36.0")]
     #[inline]
     pub fn as_mut_ptr(&mut self) -> *mut u8 {
         self as *mut str as *mut u8