1. Changes in 8.13 require `Referable`s that don't have a project
   reference to accept two arguments - `from_project` and
   `target_project`.

2. `User#to_reference` was not changed to accept the
   `target_project` (even though it is not used). Moving an issue
   containing a user reference would throw a "invalid number of
   arguments" exception.

Fixes #23662