Update /doc/howto/runner.md to use gdk.yml

What does this Merge Request do?

gdk.yml configuration has been the preferred method since October 2019. The runner setup doc has yet to be updated and still directs the user to update gitlab.yml directly.

Previous https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/runner-setup-use-gdk-yml/doc/howto/runner.md

With updates https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/doc/howto/runner.md#docker-executor

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.