Workflow failure due to Ubuntu 20 image retirement

[ureeves]

I see the workflow is failing due to github having retired their ubuntu 20 images. I also see there's a problem with simply updating to the more recent ubuntu 22 images, since they're running kernel 6.8 and as a consequence the syscall path for creating a file descriptor would no longer be tested.

I can see at least two options for addressing this problem:

1.) Modify the public API of the crate to allow the user to choose which method to use - either use the syscall or the ioctl.
2.) Introduce features, much like the one introduced in this PR, controlling which methods of creating a file descriptor are allowed.

Originally posted by @ureeves in #70 (comment)

[ureeves]

Personally, I think it might make sense to break the API of the crate by using the syscall only if the user explicitly wants it - by introducing a new UffdBuilder::use_syscall(true) function for example, or even UffdBuilder::create_with_syscall().

This would not only allow for a user to choose the method to use, but would stimulate the usage of this crate without granting the process CAP_SYS_PTRACE capability - something that the security conscious will probably appreciate.