[Rubocop] Update API/EntityFieldType

Vlad Wolanykrequested to merge
582859-rubocop-entity-field-type-update into master

What does this MR do and why?

This MR makes a small change to update API/EntityFieldType linter to validate and autocorrect the using option to ensure valid entity references. Entity refs must be constants and begin with lead colons to avoid namespace conflicts.

Related to #582859

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