Update const_slice_first_last_not_mut stable version.

Author: m-ou-se

library/core/src/slice/mod.rs

@@ -140,7 +140,7 @@ impl<T> [T] {
     /// assert_eq!(None, w.first());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.55.0")]
+    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.56.0")]
     #[inline]
     pub const fn first(&self) -> Option<&T> {
         if let [first, ..] = self { Some(first) } else { None }
@@ -178,7 +178,7 @@ impl<T> [T] {
     /// }
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
-    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.55.0")]
+    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.56.0")]
     #[inline]
     pub const fn split_first(&self) -> Option<(&T, &[T])> {
         if let [first, tail @ ..] = self { Some((first, tail)) } else { None }
@@ -218,7 +218,7 @@ impl<T> [T] {
     /// }
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
-    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.55.0")]
+    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.56.0")]
     #[inline]
     pub const fn split_last(&self) -> Option<(&T, &[T])> {
         if let [init @ .., last] = self { Some((last, init)) } else { None }
@@ -257,7 +257,7 @@ impl<T> [T] {
     /// assert_eq!(None, w.last());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.55.0")]
+    #[rustc_const_stable(feature = "const_slice_first_last_not_mut", since = "1.56.0")]
     #[inline]
     pub const fn last(&self) -> Option<&T> {
         if let [.., last] = self { Some(last) } else { None }