stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate

This is an alternative to #121920

tests/ui/mir/alignment/packed.rs

@@ -1,7 +1,7 @@
 //@ run-pass
 //@ compile-flags: -C debug-assertions
 
-#![feature(strict_provenance, pointer_is_aligned)]
+#![feature(strict_provenance)]
 
 #[repr(packed)]
 struct Misaligner {

tests/ui/structs-enums/type-sizes.rs

@@ -4,7 +4,7 @@
 #![allow(dead_code)]
 #![feature(generic_nonzero)]
 #![feature(never_type)]
-#![feature(pointer_is_aligned)]
+#![feature(pointer_is_aligned_to)]
 #![feature(strict_provenance)]
 
 use std::mem::size_of;