Resolve "Prometheus loading screen no longer seems to appear"

What does this MR do?

Fixes the broken asset paths for empty state SVG illustrations used within the ~Prometheus monitoring page.

This was not noticed before because in the GDK, incorrect asset paths are a lot more forgiving. Forgetting to include a file extension in an image path will still work locally, but in production it will not. Adding the file extension causes the url to be generated correctly.

Before:

http://localhost:3000/assets/illustrations/monitoring/unable_to_connect

After:

http://localhost:3000/assets/illustrations/monitoring/unable_to_connect-eace0f700993376a1af0fc6f50b0dfd061010d2fa0244106940007e19bbfeec9.svg

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #40285 (closed)