Automatically assign @envoyproxy/stable-maintainers to backports

@envoyproxy/stable-maintainers (or only stable release manager, but I don't believe we have a separate group for that?) should be automatically assigned all PRs sent against `release/vX.Y` branches.

We could achieve this either by:
a) Replacing `CODEOWNERS` with `* @envoyproxy/stable-maintainers`, but it doesn't look like `CODEOWNERS` are automatically assigned changes in the `release/vX.Y` branches right now.
b) Make @repokitteh auto-assign @envoyproxy/stable-maintainers for those PRs, similarly to how it auto-assigns @envoyproxy/api-shepherds for the `api/` changes.

@mattklein123 any thoughts and/or preference?