README: say kernel 4.15 is the minimum recommended

README: say kernel 4.15 is the minimum recommended

After various long discussions
(https://lists.freedesktop.org/archives/systemd-devel/2022-March/047587.html,
https://lwn.net/Articles/889610/), there is no clear answer what the minimum
version should be. Bumping the version above 3.15 doesn't allow us to make any
significant simplifications (unless we went *much* higher). In particular, even
renameat2() is not fully supported with latest kernel versions, e.g. nfs still
doesn't have it. And the bpf stuff is optional anyway. So let's just say that
4.15 is what we recommend, because it provides fairly complete cgroups-v2, but
without any removals of compat in the code.