Update 'how to become a maintainer' to add new guidelines

https://about.gitlab.com/handbook/engineering/#how-to-become-a-maintainer needs to say:

1. This is specifically about backend maintainers.
2. Anyone above the junior level can be a maintainer.
3. We suggest that people have been with GitLab for a year before considering this, to gain sufficient familiarity with the codebase.
4. Staff (or higher) positions may need less time, and they should work with their manager to drive this.

/cc @gitlab-org/maintainers/rails-backend