Ensure event type and object kind is issue for work items for issue events

When using the new work items views and GraphQL mutations, issue events are triggered with objects of type WorkItem. For this reason object_kind and event_type are work_item instead of issue which may break customer integrations.

For this reason we'll fix both attributes to issue for work items for now and then approach work item hooks as a feature.

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We build the data in lib/gitlab/data_builder/issuable.rb and depending on what kind of object we're passing (Issue or WorkItem) we get different results. So whenever we update something through work item update service (work item GraphQL) we'll pass an object of work item. object_kind is defined as issuable.class.name.underscore

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Per this internal Slack conversation and this comment.