Raise CMake minimum version to 3.20

[MrAureliusR]

This will fix a lot of build errors people are having. I picked 3.20 but this is easily change if some distros are still using older versions of CMake.

[MrAureliusR]

Converted to draft PR because this uncovered other issues with the CMake settings.

[MrAureliusR]

Aw man I forgot all about this. Unfortunately right now I don't have the time to follow up, but essentially what happened was:

• I bumped the CMAKE_MINIMUM_VERSION to 3.20
• Because of the API changes since pre-3.5 compared to 3.20, this triggered other warnings/errors
• Essentially, some syntax in CMakeLists.txt needs to be updated to reflect the changes in the API. The longer this goes, the more broken it'll get. Better to try and maintain compatibility with the latest CMake API
• Once it's building cleanly without any spurious warnings, then re-submit as a PR here. You can start with my branch or fork the current state

[boatbod]

Once you tell me you have a PR against my "dev" branch I'll take a look.
Thanks