Remove category from Internal Events event definitions - part 1

What does this MR do and why?

In gitlab-org/analytics-section/analytics-instrumentation/metric-dictionary!75 (merged) we added support for event definitions that has internal_events: true and no category.

This MR migrates event definitions from the ee directory.

The non-ee directory will be migrated in a separate MR.

This MR addresses step 3) in the below plan (copied from the issue)

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.

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Related to #435201 (closed)