Fix `User#to_reference`

Fix the method signature of `User#to_reference` so that moving an issue with a user reference does not throw a "invalid number of arguments" exception.

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.

3. The regression was introduced in [c8b2b3f7](https://gitlab.com/gitlab-org/gitlab-ce/commit/c8b2b3f7c32db873f1bebce3e3b1847ea24d235f#91fabb7ad88bd2fde6fef1c100a719c00e503047_75_79), which expects
   all `Referable`s that don't respond to `:project` to have a `to_reference`
   method that takes two arguments.

Closes #23662

See merge request !7088
Signed-off-by: Rémy Coutable <remy@rymai.me>