Revert "expr_2021 should be allowed on edition 2021 and later"

This reverts commit 73303c3.

Author: fmease

compiler/rustc_ast/src/token.rs

@@ -912,7 +912,7 @@ impl NonterminalKind {
             },
             sym::pat_param => NonterminalKind::PatParam { inferred: false },
             sym::expr => NonterminalKind::Expr,
-            sym::expr_2021 if edition() >= Edition::Edition2021 => NonterminalKind::Expr2021,
+            sym::expr_2021 if edition() >= Edition::Edition2024 => NonterminalKind::Expr2021,
             sym::ty => NonterminalKind::Ty,
             sym::ident => NonterminalKind::Ident,
             sym::lifetime => NonterminalKind::Lifetime,

tests/ui/macros/expr_2021_old_edition.rs

@@ -1,4 +1,4 @@
-//@  compile-flags: --edition=2018
+//@  compile-flags: --edition=2021
 
 // This test ensures that expr_2021 is not allowed on pre-2024 editions