Increase CMake minimum required version to 3.21

Maybe increasing the minimum CMake version is not necessary, but
the CMake documentation states:
Referencing `$<TARGET_OBJECTS>` in target_link_libraries calls worked in
versions of CMake prior to 3.21 for some cases, but was not fully
supported.

CMakeLists.txt

@@ -1,5 +1,5 @@
 message("Running CMake on dnf5...")
-cmake_minimum_required(VERSION 3.6)
+cmake_minimum_required(VERSION 3.21)
 
 
 include(VERSION.cmake)