Create 2.7 release branch

CHANGELOG.md

-# Master (will become release 2.7)
+# Release 2.7
 
 - Added fallback implementation to C++20 feature: `std::identity`.
 

dune.module

 Module: dune-common
-Version: 2.7-git
+Version: 2.7.0
 Maintainer: dune-devel@lists.dune-project.org
 Whitespace-Hook: Yes