bpf, docs: Rename legacy conformance group to packet

There could be other legacy conformance groups in the future,
so use a more descriptive name.  The status of the conformance
group in the IANA registry is what designates it as legacy,
not the name of the group.

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Link: https://lore.kernel.org/r/20240302012229.16452-1-dthaler1968@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>