Rename `API::Entities::ProjectEntity` to `API::Entities::IssuableEntity`

The following discussion from !25042 (merged) should be addressed:

• @engwan started a discussion: (+6 comments)

  This seems out of place in ProjectEntity. This is due to existing code though.

  IssueBasic, MergeRequestBasic, and MergeRequestSimple should not inherit from ProjectEntity even if they have the same fields because these objects are not projects.

  We probably should have something like IssuableEntity that's similar to ProjectEntity.

  There are also some fields in ProjectEntity that looks like they were just added for issuables. I believe projects don't have iid and project_id but I think these are added here so that the issuable entities that are inheriting from this entity gets them.

Note that API::Entities::ProjectEntity is also used in EE so an EE-port here is required.