Update the default memory limits

What does this MR do?

Update memory limit controls in config.ru.

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

Why was this MR needed?

Memory limit default in omnibus is now 450MB, source installs will now have trouble with crash loops.

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
• 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

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/37437