Resolve "Pipeline schedules i18n placeholders/text are buggy"

What does this MR do?

Correctly use gettext namespaced translation method for namespaced pipeline schedules strings.

Target is 9-3-stable because this seems to already be fixed on master.

Are there points in the code the reviewer needs to double check?

@zj @rdavila Do you think we should lint for this or do you think this will be a rare regression?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/33769