CMake: Resolve FIXMEs related to CMake < 3.20
We already have required this version for quite a while for Lagom, Ladybird and Serenity. Now that we require it in all of our CMakeLists, let's scrub for better ways of writing things.
Also some additional cleanup.
This apparently breaks running headless-browser from the ladybird app bundle on macOS... need to break out the MacBook to debug this one, that shouldn't be happening.
Seems like lagom_install_options (with the rpath stuff) is only included for LADYBIRD_IS_TOP_LEVEL, which I think isn't the case in CI
Turns out that using the install rules from lagom_install_rules.cmake don't work at all from the build directory on macOS. Needs some help from the build script to make running apps from the build dir work with that RPATH. The GN build explicitly copies all the libs, but that's a bit tedious :\