Update build process for modern libobs, add Ubuntu 22.04 compatibility, update with upstream

[dougg3]

This PR includes the following changes:

• Merge latest changes from upstream (mainly just an upgrade to version 2.9.3 with the "switching scenes reconnect" fix)
• Fix build warning with modern GCC versions
• Switch away from deprecated FFmpeg function (also to fix a build warning)
• Update the way libobs is imported to be compatible with both old and new versions of libobs
  • I reorganized the CMakeLists.txt file a little bit, now it requires a newer version of CMake. Ubuntu 18.04 users will need to upgrade their CMake.
• Add compatibility notes about 22.04 and update automated build to also build for 22.04

[dougg3]

I'm not sure what this change in the build process means for users of other distros, but as long as CMake is able to find their libobs, either the old way or the new way, it should work. OBS's Ubuntu packages provide the headers and CMake files for libobs. If other distros do the same thing, it should work. If not, you need to install the libobs development headers/CMake files before this plugin will build.