Update 404 and 403 pages

What does this MR do?

Updates the 404 and 403 pages.

Notes:

• Closes #25955 (closed)

Why was this MR needed?

#25955 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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
• 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?

#25955 (closed)

Screenshots

404 Page

404 Page - Responsive

403 Page

403 Page - Responsive

TODO

• Add screenshots of new pages
• Generate a fresh gitlab.pot
• Research other uses of public/404.html