Rename {create,emit}_warning as {create,emit}_warn.

For consistency with `warn`/`struct_warn`, and also `{create,emit}_err`,
all of which use an abbreviated form.

compiler/rustc_attr/src/builtin.rs

@@ -621,7 +621,7 @@ pub fn eval_condition(
                 }
             };
             let Some(min_version) = parse_version(*min_version) else {
-                dcx.emit_warning(session_diagnostics::UnknownVersionLiteral { span: *span });
+                dcx.emit_warn(session_diagnostics::UnknownVersionLiteral { span: *span });
                 return false;
             };