docs: update BPF features list (tracepoints to debug eBPF were removed)

Tracepoints for debugging eBPF were removed from the kernel (some other
mechanisms were being used instead, and they were not used often enough
to get proper maintenance). Let's specify this on the related line in
BPF features doc.

docs/kernel-versions.md

@@ -57,7 +57,7 @@ BPF attached to cgroups for socket filtering | 4.10 | [`0e33661de493`](https://g
 Lightweight tunnel encapsulation | 4.10 | [`3a0af8fd61f9`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3a0af8fd61f90920f6fa04e4f1e9a6a73c1b4fd2)
 **e**BPF support for `xt_bpf` module (iptables) | 4.10 | [`2c16d6033264`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=2c16d60332643e90d4fa244f4a706c454b8c7569)
 BPF program tag | 4.10 | [`7bd509e311f4`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7bd509e311f408f7a5132fcdde2069af65fa05ae)
-Tracepoints to debug BPF | 4.11 | [`a67edbf4fb6d`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=a67edbf4fb6deadcfe57a04a134abed4a5ba3bb5)
+Tracepoints to debug BPF | 4.11 (removed in 4.18) | [`a67edbf4fb6d`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=a67edbf4fb6deadcfe57a04a134abed4a5ba3bb5) [`4d220ed0f814`](https://git.kernel.org/cgit/linux/kernel/git/bpf/bpf-next.git/commit/?id=4d220ed0f8140c478ab7b0a14d96821da639b646)
 Testing / benchmarking BPF programs | 4.12 | [`1cf1cae963c2`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=1cf1cae963c2e6032aebe1637e995bc2f5d330f4)
 BPF programs and maps IDs | 4.13 | [`dc4bb0e23561`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=dc4bb0e2356149aee4cdae061936f3bbdd45595c)
 BPF support for `sock_ops` | 4.13 | [`40304b2a1567`](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=40304b2a1567fecc321f640ee4239556dd0f3ee0)