Update feature_category of workers

What does this MR do and why?

Addresses #336991 (closed).

This MR updates the feature category of the following workers:

Worker	Current Feature Category	New Feature Category
onboarding_progress_worker.rb	product_analytics	onboarding
onboarding_pipeline_created_worker.rb	subgroups	onboarding
onboarding_issue_created_worker.rb	issue_tracking	onboarding
in_product_marketing_emails_worker.rb	subgroups	experimentation_activation
onboarding_user_added_worker.rb	users	onboarding

With these changes the ownership of the updated workers are moved to Growth Groups.

MR acceptance checklist

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