Supported Linux kernel versions

Description

Currently, the README states that youki requires Linux kernel 5.3 or later.

• https://github.com/youki-dev/youki#requires

However, youki uses various Linux kernel features and syscalls that have been introduced or changed across different kernel versions. As the implementation evolves, it is not always clear which kernel versions are actually supported or whether the minimum supported kernel version is continuously tested.

For example, mount_setattr was introduced in Linux 5.12.

For reference, a similar discussion is taking place in runc:

https://github.com/opencontainers/runc/issues/5113

What we should consider

• Define the overall minimum supported Linux kernel version.
• Define feature-specific kernel version requirements.
• Clarify how the minimum supported kernel version is tested and maintained.