Auto merge of rust-lang#51144 - tmccombs:unix-epoch-stable, r=Mark-Simulacrum

Stabilize SystemTime::UNIX_EPOCH

Fixes rust-lang#49502

Author: bors

src/libstd/time.rs

@@ -271,15 +271,14 @@ impl SystemTime {
     /// # Examples
     ///
     /// ```no_run
-    /// #![feature(assoc_unix_epoch)]
     /// use std::time::SystemTime;
     ///
     /// match SystemTime::now().duration_since(SystemTime::UNIX_EPOCH) {
     ///     Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
     ///     Err(_) => panic!("SystemTime before UNIX EPOCH!"),
     /// }
     /// ```
-    #[unstable(feature = "assoc_unix_epoch", issue = "49502")]
+    #[stable(feature = "assoc_unix_epoch", since = "1.28.0")]
     pub const UNIX_EPOCH: SystemTime = UNIX_EPOCH;
 
     /// Returns the system time corresponding to "now".