CMake: Drop minimum version to 3.10 (#682)

Partially reverts 7988f1a
to restore compatibility with Ubuntu Bionic system CMake.
(However, the incompatibility between CMake < 3.12 and 
the bundled jsoncpp build targets still exists, despite this change.)

CMakeLists.txt

@@ -24,7 +24,7 @@
 # along with OpenShot Library. If not, see <http://www.gnu.org/licenses/>.
 ################################################################################
 
-cmake_minimum_required(VERSION 3.12...3.20 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.10...3.20 FATAL_ERROR)
 
 message("\
 -----------------------------------------------------------------