Replace some usage of #[rustc_on_unimplemented] with

`#[diagnostic::on_unimplemented]`

This commit replaces those `#[rustc_on_unimplemented]` attributes with
their equivalent `#[diagnostic::on_unimplemented]` where this is
supported (So no filter or any extended option)

library/core/src/lib.rs

@@ -218,6 +218,7 @@
 #![feature(const_trait_impl)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
+#![feature(diagnostic_namespace)]
 #![feature(doc_cfg)]
 #![feature(doc_cfg_hide)]
 #![feature(doc_notable_trait)]