Fix the last-ditch memory killer pgroup SIGKILL

What does this MR do?

Fixes one more instance of passing a negative number as a string to Process.kill

This one is just the last-ditch kill -9 <pgroup> built into the sidekiq memory killer. I suspect we don't touch it very often, but it's important that it does the right thing when we do!

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25866

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
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