Change CMake minimum version to 3.14.0

[triuk]

As the title says, I suggest changing the line at CMakeFiles.txt to:
cmake_minimum_required(VERSION 3.14.0 FATAL_ERROR)
to avoid the OpenGL find_package warning during configuration.

[rajkundu]

Which OpenGL find_package warnings are you getting, and what system are you on? I'm on macOS 12.5 with Intel and I don't see any warnings. Here is my VS Code output:

[main] Configuring folder: imgui-vtk 
[proc] Executing command: /usr/local/bin/cmake --no-warn-unused-cli -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE -DCMAKE_BUILD_TYPE:STRING=Release -DCMAKE_C_COMPILER:FILEPATH=/usr/bin/clang -DCMAKE_CXX_COMPILER:FILEPATH=/usr/bin/clang++ -S/Users/raj/code/imgui-vtk -B/Users/raj/code/imgui-vtk/build -G "Unix Makefiles"
[cmake] Not searching for unused variables given on the command line.
[cmake] -- Found gl3w source files
[cmake] -- Including Cocoa support
[cmake] -- VTK Version: 9.2.2
[cmake] -- Configuring done
[cmake] -- Generating done
[cmake] -- Build files have been written to: /Users/raj/code/imgui-vtk/build