libbpf: Reject legacy 'maps' ELF section

Add explicit error message if BPF object file is still using legacy BPF
map definitions in SEC("maps"). Before this change, if BPF object file
is still using legacy map definition user will see a bit confusing:

  libbpf: elf: skipping unrecognized data section(4) maps
  libbpf: prog 'handler': bad map relo against 'server_map' in section 'maps'

Now libbpf will be explicit about rejecting "maps" ELF section:

  libbpf: elf: legacy map definitions in 'maps' section are not supported by libbpf v1.0+

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220803214202.23750-1-andrii@kernel.org