Rollup merge of #67480 - rossmacarthur:fix-41260-avoid-issue-0-part-2, r=Centril

Require issue = "none" over issue = "0" in unstable attributes

These changes make the use of `issue = "none"` required in unstable attributes throughout the compiler.

Notes:
- #66299 is now in beta so `issue = "none"` is accepted.
- The `tidy` tool now fails on `issue = "0"`.
- Tests that used `issue = "0"` were changed to use `issue = "none"`, except for _one_ that asserts `issue = "0"` can still be used.
- The compiler still allows `issue = "0"` because some submodules require it, this could be disallowed once these are updated.

Resolves #41260

r? @varkor

src/libcore/ptr/mod.rs

@@ -89,7 +89,7 @@ mod non_null;
 pub use non_null::NonNull;
 
 mod unique;
-#[unstable(feature = "ptr_internals", issue = "0")]
+#[unstable(feature = "ptr_internals", issue = "none")]
 pub use unique::Unique;
 
 mod const_ptr;