Fix `User#to_reference`

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

app/models/user.rb

@@ -309,7 +309,7 @@ class User < ActiveRecord::Base
     username
   end
-  def to_reference(_from_project = nil)
+  def to_reference(_from_project = nil, _target_project = nil)
     "#{self.class.reference_prefix}#{username}"
   end

spec/services/issues/move_service_spec.rb

@@ -207,10 +207,10 @@ describe Issues::MoveService, services: true do
         end
       end
-      describe 'rewritting references' do
+      describe 'rewriting references' do
         include_context 'issue move executed'
-        context 'issue reference' do
+        context 'issue references' do
           let(:another_issue) { create(:issue, project: old_project) }
           let(:description) { "Some description #{another_issue.to_reference}" }
@@ -219,6 +219,16 @@ describe Issues::MoveService, services: true do
               .to eq "Some description #{old_project.to_reference}#{another_issue.to_reference}"
           end
         end
+
+        context "user references" do
+          let(:another_issue) { create(:issue, project: old_project) }
+          let(:description) { "Some description #{user.to_reference}" }
+
+          it "doesn't throw any errors for issues containing user references" do
+            expect(new_issue.description)
+              .to eq "Some description #{user.to_reference}"
+          end
+        end
       end
       context 'moving to same project' do