libbpf 1.0 not upgradable yet (headers madness)

FWIW libbpf 1.0 released recently has the same problem as #13338 (closed) had: we'll need linux-headers 6.0 (not released yet!) for it to be usable... :/

see https://github.com/xdp-project/xdp-tools/issues/220 even if it's slightly different as they also vendor their own kernel headers

I feel we really want to address this and use libbpf's linux/btf.h and friends over linux-headers' when possible, but I still don't have any idea on how to do that properly. This issue will probably keep coming up on every release unless we can address this.

Well, I guess we can just wait it out unless a better solution pops up -- please don't update libbpf for now :) cc @acj @Leo @ncopa @psykose