Stabilize fs_set_times

This stabilizes both functions under this feature gate:

* `std::fs::set_times`
* `std::fs::set_times_nofollow`

library/std/src/fs.rs

diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs
index 3b84997..58874a2 100644
--- a/library/std/src/fs.rs
+++ b/library/std/src/fs.rs
@@ -443,7 +443,6 @@ fn inner(path: &Path, contents: &[u8]) -> io::Result<()> {
 /// # Examples
 ///
 /// ```no_run
-/// #![feature(fs_set_times)]
 /// use std::fs::{self, FileTimes};
 /// use std::time::SystemTime;
 ///
@@ -456,7 +455,7 @@ fn inner(path: &Path, contents: &[u8]) -> io::Result<()> {
 ///     Ok(())
 /// }
 /// ```
-#[unstable(feature = "fs_set_times", issue = "147455")]
+#[stable(feature = "fs_set_times", since = "CURRENT_RUSTC_VERSION")]
 #[doc(alias = "utimens")]
 #[doc(alias = "utimes")]
 #[doc(alias = "utime")]
@@ -484,7 +483,6 @@ pub fn set_times<P: AsRef<Path>>(path: P, times: FileTimes) -> io::Result<()> {
 /// # Examples
 ///
 /// ```no_run
-/// #![feature(fs_set_times)]
 /// use std::fs::{self, FileTimes};
 /// use std::time::SystemTime;
 ///
@@ -497,7 +495,7 @@ pub fn set_times<P: AsRef<Path>>(path: P, times: FileTimes) -> io::Result<()> {
 ///     Ok(())
 /// }
 /// ```
-#[unstable(feature = "fs_set_times", issue = "147455")]
+#[stable(feature = "fs_set_times", since = "CURRENT_RUSTC_VERSION")]
 #[doc(alias = "utimensat")]
 #[doc(alias = "lutimens")]
 #[doc(alias = "lutimes")]