Remove category from Internal Events event definitions

What does this MR do and why?

As a last step to solve #435201 (closed) this MR addresses point 5) and 6) of the proposed solution.

1. Allow event definitions to have an optional internal_events property and add it to a single event definition
2. Make the Metric Dictionary use the internal_events property instead of category if internal_events is present
3. Make category optional in the event definition schema. Migrate first batch of Internal Events event definitions to have no category and internal_events: true
4. Migrate the remaining event definitions
5. Modify the generator to output event definitions with internal_events: true instead of category: InternalEventTracking.
6. Modify the schema to make sure category: InternalEventTracking is not allow and migrate any event definition created after step 4 above.

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a new event with the generator and verify that it doesn't have a category but has internal_events: true.
2. Add category: MyCategory to gitlab/config/events/p_analytics_ci_cd_deployment_frequency.yml and see that bundle exec rspec ./spec/lib/gitlab/tracking/event_definition_validate_all_spec.rb fails.

Related to #435201 (closed)