Fix stability attributes.

Author: bjoernager

library/core/src/char/methods.rs

@@ -673,7 +673,7 @@ impl char {
     /// 'ß'.encode_utf8(&mut b);
     /// ```
     #[stable(feature = "unicode_encode_char", since = "1.15.0")]
-    #[rustc_const_stable(feature = "const_char_encode_utf8", issue = "130512")]
+    #[rustc_const_unstable(feature = "const_char_encode_utf8", issue = "130512")]
     #[inline]
     pub const fn encode_utf8(self, dst: &mut [u8]) -> &mut str {
         // SAFETY: `char` is not a surrogate, so this is valid UTF-8.
@@ -1762,6 +1762,7 @@ const fn len_utf8(code: u32) -> usize {
 /// Panics if the buffer is not large enough.
 /// A buffer of length four is large enough to encode any `char`.
 #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
+#[rustc_const_unstable(feature = "const_char_encode_utf8", issue = "130512")]
 #[doc(hidden)]
 #[inline]
 pub const fn encode_utf8_raw(code: u32, dst: &mut [u8]) -> &mut [u8] {