chore(RingTheory/ClassGroup): deprecate module (leanprover-community#40157)

Follow-up for leanprover-community#38127.

Author: riccardobrasca

Mathlib.lean

@@ -6399,6 +6399,7 @@ public import Mathlib.RingTheory.Bialgebra.SymmetricAlgebra
 public import Mathlib.RingTheory.Bialgebra.TensorProduct
 public import Mathlib.RingTheory.Binomial
 public import Mathlib.RingTheory.ChainOfDivisors
+public import Mathlib.RingTheory.ClassGroup
 public import Mathlib.RingTheory.ClassGroup.Basic
 public import Mathlib.RingTheory.ClassGroup.ExtendedHom
 public import Mathlib.RingTheory.Coalgebra.Basic

Mathlib/RingTheory/ClassGroup.lean

@@ -0,0 +1,10 @@
+/-
+Copyright (c) 2021 Anne Baanen. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Anne Baanen
+-/
+module -- shake: keep-all
+
+public import Mathlib.RingTheory.ClassGroup.Basic
+
+deprecated_module (since := "2026-06-02")