cmake: Add upper version for compatibility

Limiting the supported version enables CMake to not enable bleeding-edge
features even on more recent installed versions and ensures no sudden
breaking changes.

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.16)
+cmake_minimum_required(VERSION 3.16...3.21)
 
 # Change obs-plugintemplate to your plugin's name in a machine-readable format
 # (e.g.: obs-myawesomeplugin) and set