Extract strings to gettext

What does this MR do?

Make strings in app/views/notify/closed_issue_email.html.haml app/views/notify/closed_issue_email.text.haml translatable.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

Closes #61960 (closed)