Auto merge of #103858 - Mark-Simulacrum:bump-bootstrap, r=pietroalbini

Bump bootstrap compiler to 1.66

This PR:

- Bumps version placeholders to release
- Bumps to latest beta
- cfg-steps code

r? `@pietroalbini`

compiler/rustc_errors/src/diagnostic.rs

@@ -64,8 +64,7 @@ impl<'source> Into<FluentValue<'source>> for DiagnosticArgValue<'source> {
 
 /// Trait implemented by error types. This should not be implemented manually. Instead, use
 /// `#[derive(Subdiagnostic)]` -- see [rustc_macros::Subdiagnostic].
-#[cfg_attr(bootstrap, rustc_diagnostic_item = "AddSubdiagnostic")]
-#[cfg_attr(not(bootstrap), rustc_diagnostic_item = "AddToDiagnostic")]
+#[rustc_diagnostic_item = "AddToDiagnostic"]
 pub trait AddToDiagnostic
 where
     Self: Sized,