Stop writing to update_at on audit_events

We are dropping this column as this is an append-only table. Even
though the column has been marked as ignored (via `IgnoreableColumn`),
we still miss the use case where attributes are updated directly via
`Gitlab::Database.bulk_insert`.

app/models/audit_event.rb

@@ -4,7 +4,7 @@ class AuditEvent < ApplicationRecord
   include CreatedAtFilterable
   include IgnorableColumns
 
-  ignore_column :updated_at, remove_with: '13.3', remove_after: '2020-08-22'
+  ignore_column :updated_at, remove_with: '13.4', remove_after: '2020-09-22'
 
   serialize :details, Hash # rubocop:disable Cop/ActiveRecordSerialize
 

ee/app/services/ee/audit_events/repository_push_audit_event_service.rb

@@ -16,7 +16,6 @@ module EE
       def attributes
         base_payload.merge(type: SecurityEvent.to_s,
                            created_at: DateTime.current,
-                           updated_at: DateTime.current,
                            details: @details.to_yaml)
       end
 

ee/spec/services/ee/audit_events/bulk_insert_service_spec.rb

@@ -22,7 +22,6 @@ RSpec.describe EE::AuditEvents::BulkInsertService do
       entity_type: entity_type,
       type: 'SecurityEvent',
       created_at: timestamp,
-      updated_at: timestamp,
       details: {
         updated_ref: 'master',
         author_name: user.name,

ee/spec/services/ee/audit_events/repository_push_audit_event_service_spec.rb

@@ -24,7 +24,6 @@ RSpec.describe EE::AuditEvents::RepositoryPushAuditEventService do
         entity_type: entity_type,
         type: 'SecurityEvent',
         created_at: timestamp,
-        updated_at: timestamp,
         ip_address: '127.0.0.1',
         details: {
           updated_ref: updated_ref,