Map jira users for server version by 'key' attribute

In Jira Cloud user identifier is returned in 'accountId' attribute
while in Jira Server it is being returned in 'key' attribute. This
fixes the user mapping for jira issues import from Jira Server instance.

Changelog: fixed

lib/gitlab/jira_import/issue_serializer.rb

@@ -52,9 +52,10 @@ module Gitlab
       end
 
       def map_user_id(jira_user)
-        return unless jira_user&.dig('accountId')
+        jira_user_identifier = jira_user&.dig('accountId') || jira_user&.dig('key')
+        return unless jira_user_identifier
 
-        Gitlab::JiraImport.get_user_mapping(project.id, jira_user['accountId'])
+        Gitlab::JiraImport.get_user_mapping(project.id, jira_user_identifier)
       end
 
       def reporter

spec/lib/gitlab/jira_import/issue_serializer_spec.rb

@@ -192,6 +192,19 @@ RSpec.describe Gitlab::JiraImport::IssueSerializer do
             expect(subject[:assignee_ids]).to be_nil
           end
         end
+
+        context 'with jira server response' do
+          let(:assignee) { double(attrs: { 'displayName' => 'Solver', 'key' => '1234' }) }
+
+          context 'when assignee maps to a valid GitLab user' do
+            it 'sets the issue assignees to the mapped user' do
+              expect(Gitlab::JiraImport).to receive(:get_user_mapping).with(project.id, '1234')
+                                                                      .and_return(user.id)
+
+              expect(subject[:assignee_ids]).to eq([user.id])
+            end
+          end
+        end
       end
     end