Follow-up from "Refactor Epic mutations base"

The following discussion from !25991 (merged) should be addressed:

• @egrieff started a discussion: (+3 comments)

  Note: The type here is changing from ID_TYPE to STRING_TYPE, but I think it would be ok for consistency with other iid arguments 🤔

We should modify IIDs to use ID_TYPE rather than STRING_TYPE for consistency.

from @jarka's comment:

I personally would use ID_TYPE as we use it for all :id arguments. I think we should change other iids (not in this MR) rather than using STRING_TYPE here