Bump minimum CMake version to 3.6

[trinitronx]

Fix building on CMake 4.0.2, which shows error:

CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.


-- Configuring incomplete, errors occurred!

Related to:

• lib/zlib: Bump minimum CMake version to 3.6 AppImageCommunity/zsync2#82
• Update minimum required CMake version to 3.5 TheAssassin/AppImageLauncher#717