Require valid product_group in event definition schema

What does this MR do and why?

Now that all event definitions have a valid product_group (one that is present in product_groups.json), we want to use schema validation to avoid new invalid values from being introduced.

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Before	After

How to set up and validate locally

1. Set product_group: wacky_widgets in ee/config/events/20210915205139_packages_cancel_delete_package.yml
2. Run rspec ./spec/lib/gitlab/tracking/event_definition_validate_all_spec.rb and see the spec fail.

Related to #454099