Update linux kernel

Our documentation and tests currently run against Firecracker's getting started kernel which is 4.14 which was released in 2017. We're starting to see important feature drift such as modern distros enabling cgroups v2 by default which is not supported by 4.14.

We should at least test against 5.10 which is the other kernel that Firecracker continuously tests against: https://github.com/firecracker-microvm/firecracker/blob/main/docs/kernel-policy.md

Maybe we should match Firecracker and continuously test against Firecracker's supported kernel versions? In the future, that could allow us to start doing regression tests against new kernels early while still having good test coverage for the kernels that our customers use.