Avoid saving author object in DB

Since the introduction of `UnauthenticatedAuthor` object, failed login
event has been persisted as a serialized Ruby object in DB.

!ruby/object:Gitlab::Audit::UnauthenticatedAuthor

This causes undesirable coupling with application code and hinders
future DB migration.

ee/app/services/ee/audit_event_service.rb

@@ -90,7 +90,7 @@ module EE
       @details = {
         failed_login: auth.upcase,
         author_name: @author.name,
-        target_details: @author,
+        target_details: @author.name,
         ip_address: ip
       }
 

ee/changelogs/unreleased/216577-fix-ruby-object-in-audit-event.yml

+---
+title: Avoid saving author object in audit_events table
+merge_request: 31456
+author:
+type: fixed

ee/spec/services/audit_event_service_spec.rb

@@ -249,6 +249,10 @@ describe AuditEventService do
       expect(event.details[:author_name]).to eq(author_name)
     end
 
+    it 'has the right target_details' do
+      expect(event.details[:target_details]).to eq(author_name)
+    end
+
     it 'has the right auth method for OAUTH' do
       oauth_service = described_class.new(author_name, nil, ip_address: ip_address, with: 'ldap')
       event = oauth_service.for_failed_login.unauth_security_event