docs/bpf: Default cpu version changed from v1 to v3 in llvm 20

The default cpu version is changed from v1 to v3 in llvm version 20.
See [1] for more detailed reasoning. Update bpf_devel_QA.rst so
developers can find such information easily.

  [1] https://github.com/llvm/llvm-project/pull/107008



Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250612043049.2411989-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>