QA: Set sandbox group visibility to public

What does this MR do?

Sets the sandbox group visibility to public when executing qa/specs/features/api/2_plan/ee_epics_milestone_dates_spec.rb

It seems that the API creates a group as private by default, which means that if the sandbox group is created as private none of the subgroups can be public, so every subsequent test would fail.

What are the relevant issue numbers?

No issue. Discovered while verifying another MR. See failed tests: https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/109334957

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?