Add timezone information to the datetime select labels

What does this MR do?

Adds missing timezone information (UTC default) to the broadcast message datetime selects label.
This information was not transparent to a user previously and only shown on existing messages.

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

Externalization correct?

Why was this MR needed?

See #23389 (closed)

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #23389 (closed).