[!739] Bump version to 2.8

Merge branch 'feature/bump-to-2.8' into 'master'

See merge request [!739]

  [!739]: Nonecore/dune-common/merge_requests/739

CHANGELOG.md

-# Master (will become release 2.7)
+# Master (will become release 2.8)
+
+# Release 2.7
 
 - Added fallback implementation to C++20 feature: `std::identity`.
 

dune.module

 Module: dune-common
-Version: 2.7-git
+Version: 2.8-git
 Maintainer: dune-devel@lists.dune-project.org
 Whitespace-Hook: Yes