Improve gdk stop/start service order

What does this Merge Request do and why?

This MR aims to improve the order in which services are stopped/started to ensure a more reliable experience.

Before

$ gdk start
ok: run: ./services/gitlab-workhorse: (pid 89840) 0s, normally down
ok: run: ./services/postgresql: (pid 89843) 0s, normally down
ok: run: ./services/rails-web: (pid 89847) 0s, normally down
ok: run: ./services/redis: (pid 89845) 0s, normally down
ok: run: ./services/sshd: (pid 89849) 0s, normally down
ok: run: ./services/webpack: (pid 89850) 0s, normally down
ok: run: ./services/gitlab-docs: (pid 89839) 0s, normally down
ok: run: ./services/grafana: (pid 89841) 0s, normally down
ok: run: ./services/jaeger: (pid 89842) 0s, normally down
ok: run: ./services/praefect-gitaly-0: (pid 89848) 0s, normally down
ok: run: ./services/rails-background-jobs: (pid 89846) 0s, normally down
ok: run: ./services/praefect: (pid 90328) 1s, normally down

=> GitLab will be available at http://gdk.test:3000 shortly.
=> GitLab Docs will be available at http://gdk.test:3005 shortly.

$ gdk stop
ok: down: ./services/gitlab-docs: 0s
ok: down: ./services/gitlab-workhorse: 0s
ok: down: ./services/grafana: 0s
ok: down: ./services/postgresql: 0s
ok: down: ./services/praefect: 0s
ok: down: ./services/praefect-gitaly-0: 0s
ok: down: ./services/rails-web: 0s
ok: down: ./services/redis: 0s
ok: down: ./services/sshd: 0s
ok: down: ./services/webpack: 0s
ok: down: ./services/rails-background-jobs: 0s
ok: down: ./services/jaeger: 0s

✅️ All services have been stopped!

After

$ gdk start
ok: run: ./services/postgresql: (pid 87078) 1s, normally down
ok: run: ./services/redis: (pid 87160) 0s, normally down
ok: run: ./services/praefect: (pid 87390) 1s, normally down
ok: run: ./services/praefect-gitaly-0: (pid 87391) 1s, normally down
ok: run: ./services/gitlab-docs: (pid 87426) 0s, normally down
ok: run: ./services/gitlab-workhorse: (pid 87427) 0s, normally down
ok: run: ./services/grafana: (pid 87428) 0s, normally down
ok: run: ./services/jaeger: (pid 87429) 0s, normally down
ok: run: ./services/rails-background-jobs: (pid 87430) 0s, normally down
ok: run: ./services/rails-web: (pid 87431) 0s, normally down
ok: run: ./services/sshd: (pid 87433) 0s, normally down
ok: run: ./services/webpack: (pid 87432) 0s, normally down

=> GitLab will be available at http://gdk.test:3000 shortly.
=> GitLab Docs will be available at http://gdk.test:3005 shortly.

$ gdk stop
ok: down: ./services/gitlab-docs: 0s
ok: down: ./services/gitlab-workhorse: 0s
ok: down: ./services/grafana: 0s
ok: down: ./services/rails-web: 0s
ok: down: ./services/sshd: 0s
ok: down: ./services/webpack: 0s
ok: down: ./services/jaeger: 0s
ok: down: ./services/rails-background-jobs: 0s
ok: down: ./services/praefect: 0s
ok: down: ./services/praefect-gitaly-0: 0s
ok: down: ./services/redis: 0s
ok: down: ./services/postgresql: 0s

✅️ All services have been stopped!

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.