Increase the memory limits used in the unicorn killer

What does this MR do?

Increases the unicorn killer memory limits. These were changed a while back in omnibus here: omnibus-gitlab@c6a98444

And in the documentation here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6016

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

Why was this MR needed?

Source installs and the cloud-native charts are seeing more memory exceeded worker kills than needed.

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
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes: https://gitlab.com/gitlab-com/infrastructure/issues/19