bin/background_jobs: use pgroups

What does this MR do?

Modifies the bin/background_jobs script to start sidekiq in its own process group if the chpst program is available.

This script is used to run sidekiq in from-source and GDK installations. Putting sidekiq into a pgroup means we can reliably kill it, and all its children, at the same time.

I don't think we use it in cloud native setups. I need to check whether that's the case or not, although it shouldn't matter either way.

If chpst isn't installed, we fall back to the original behaviour.

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
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #40396 (closed)