message: api: remove vNalphaM as style guide recommendation.
  We want to have just vN and vNalpha going forward as part of the v3
  work.
diff: @@ -47,9 +47,8 @@ In addition, the following conventions should be followed:
  * Protos for configs and services that are not implemented immediately in
  Envoy, or are under active design and development should be versioned
  - "v2alpha". If several iterations of the alpha API are expected, then versions
  - "v2alpha1", "v2alpha2", and so on are preferred. Alpha-versioned protos are
  - considered experimental and are not required to preserve compatibility.
  + "vNalpha". See the [stable API versioning
  + policy](https://github.com/envoyproxy/envoy/issues/6271).
  * Every proto directory should have a `README.md` describing its content. See
  for example [envoy.service](envoy/service/README.md).
  