Resolve "First request after GDK server started is slow"

What does this MR do?

Take the advantage of warmup feature in Rack.

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

This is not really improving performance, but moving the loading times around. Is this making the experience better, or worse?

Why was this MR needed?

Make the first request faster. (Before this patch, the first request is 100% 502 for me)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #38930 (closed)