Be more vigorous killing workers when shutting down sidekiq-cluster

What does this MR do?

In https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8511 and gitlab-com/gl-infra/production#1309 (closed) we have seen cases where sidekiq-cluster has been asked to restart, but it has left one or more sidekiq worker processes in a stuck/hung state burning CPU.

With this, sidekiq-cluster now waits for the worker processes to terminate themselves cleanly (and push jobs back onto the queue) and another few seconds beyond, then if any processes remain, kills them hard (:KILL signal) before exiting as usual (allowing whater process monitor is in place to restart sidekiq-cluster)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers