Improve grammar in 'gdk start' and 'gdk stop' messages

What does this Merge Request do?

When I run gdk start, it says:

GitLab available at http://127.0.0.1:3000 shortly.

…which is somewhat ungrammatical (the verb is missing); this MR changes it to:

GitLab will be available at http://127.0.0.1:3000 shortly.

Also, when I run gdk stop, it says:

Shutting all services:
…
All services are shutdown!

…which sounds awkward to me; this MR changes it to:

Shutting down all services:
…
All services have been shut down!

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up. — N/A
• 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. — N/A