Update changelog file for release 2.10

CHANGELOG.md

@@ -3,7 +3,7 @@ SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE
 SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
 -->
 
-# Master (will become release 2.10)
+# Release 2.10
 
 - Improve testing support on Laplacian matrices with an optional diagonal regularization parameter.