Rollup merge of #68984 - ecstatic-morse:const-u8-is-ascii, r=sfackler

Make `u8::is_ascii` a stable `const fn`

`char::is_ascii` was already stabilized as `const fn` in #55278, so there is no reason for `u8::is_ascii` to go through an unstable period.

cc @rust-lang/libs

src/test/ui/consts/is_ascii.rs

@@ -3,7 +3,13 @@
 static X: bool = 'a'.is_ascii();
 static Y: bool = 'ä'.is_ascii();
 
+static BX: bool = b'a'.is_ascii();
+static BY: bool = 192u8.is_ascii();
+
 fn main() {
     assert!(X);
     assert!(!Y);
+
+    assert!(BX);
+    assert!(!BY);
 }