bump version to 2.5.6

Author: daschuer

CMakeLists.txt

@@ -228,7 +228,7 @@ elseif(APPLE)
   endif()
 endif()
 
-project(mixxx VERSION 2.5.4)
+project(mixxx VERSION 2.5.6)
 enable_language(C CXX)
 # Work around missing version suffixes support https://gitlab.kitware.com/cmake/cmake/-/issues/16716
 set(MIXXX_VERSION_PRERELEASE "") # set to "alpha" "beta" or ""