Bump shrink_to stabilization to Rust 1.56

library/alloc/src/string.rs

@@ -1110,7 +1110,7 @@ pub fn shrink_to_fit(&mut self) {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
-    #[stable(feature = "shrink_to", since = "1.55.0")]
+    #[stable(feature = "shrink_to", since = "1.56.0")]
     pub fn shrink_to(&mut self, min_capacity: usize) {
         self.vec.shrink_to(min_capacity)
     }