Externalize strings in admin controllers

What does this MR do?

This is one of many smaller MRs that ensures untranslated strings which have been detected by rubocop-i18n (see also https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24737) are externalized.

It externalizes strings in files located under app/controllers/admin which have been detected by rubocop-i18n.

Note: rubocop-i18n is not part of our codebase yet but will eventually be added in a separate MR.

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10336

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57968

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer