Rollup merge of rust-lang#100337 - camelid:stabilize-io_read_to_strin…

…g, r=JohnTitor

Stabilize `std::io::read_to_string`

Closes rust-lang#80218. 🎉

**Blocked on FCP finishing**

library/std/src/io/mod.rs

@@ -1028,8 +1028,6 @@ pub trait Read {
 /// # Examples
 ///
 /// ```no_run
-/// #![feature(io_read_to_string)]
-///
 /// # use std::io;
 /// fn main() -> io::Result<()> {
 ///     let stdin = io::read_to_string(io::stdin())?;
@@ -1038,7 +1036,7 @@ pub trait Read {
 ///     Ok(())
 /// }
 /// ```
-#[unstable(feature = "io_read_to_string", issue = "80218")]
+#[stable(feature = "io_read_to_string", since = "1.65.0")]
 pub fn read_to_string<R: Read>(mut reader: R) -> Result<String> {
     let mut buf = String::new();
     reader.read_to_string(&mut buf)?;