CODING_STYLE: document how to handle kernel compat

CODING_STYLE: document how to handle kernel compat

Let's define a way how to mark codepaths that are subject to
deletion once the kernel baseline reaches a certain version, to make it
easier to find these cases.

WHile we are at it, introuce a whole section in CODING_STYLE about
kernel version compat.

I followed the new scheme in #39621, but we can merge the coding style
guidelines on this already.