libbpf: Mark bpf_object__open_buffer() API deprecated

Deprecate bpf_object__open_buffer() API in favor of the unified
opts-based bpf_object__open_mem() API.

Signed-off-by: Christy Lee <christylee@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220125005923.418339-2-christylee@fb.com