Stabilize const-calling existing const-fns in std

Fixes #46038

src/libstd/sync/once.rs

@@ -156,7 +156,6 @@ struct Finish {
 impl Once {
     /// Creates a new `Once` value.
     #[stable(feature = "once_new", since = "1.2.0")]
-    #[rustc_const_unstable(feature = "const_once_new")]
     pub const fn new() -> Once {
         Once {
             state: AtomicUsize::new(INCOMPLETE),