ux: Add a check for older kernels

[javierhonduco]

Add a check for older, unsupported kernels. I believe the minimum we support is 5.3 (as it introduced bounded loops) + BTF. More context in this comment. For context, we have experimented with supporting older kernels (~4.20) but unrolling the loops resulted in significantly more code that will result in more tail calls, which aren't free. The current tradeoff we make here is only supporting "newer" (~4 years old) kernels but having a better performance without having to maintain tests and other infra for the two variants.