Prefer GDK::Output.divider over GDK.puts_separator

What does this merge request do and why?

This reduces GDK interface.

Contributes to #2125.

How to set up and validate locally

# On main
gdk install -fail-arg > before

# On this branch
gdk install -fail-arg > after

# No changes
diff -u before after

$ gdk install -fail-arg
make: ail-arg: No such file or directory
make: *** No rule to make target 'ail-arg'.  Stop.
❌️ ERROR: Failed to install.
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You can try the following that may be of assistance:

- Run 'gdk doctor'.

- Visit the troubleshooting documentation:
  https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/troubleshooting/index.md.
- Visit https://gitlab.com/gitlab-org/gitlab-development-kit/-/issues to
  see if there are known issues.

- Run 'gdk reset-data' if appropriate.
- Run 'gdk pristine' which will restore your GDK to a pristine state.
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Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.