DEV: prevent initialized constant warning

Was tired of seeing the following warnings in the logs

```
/discourse/app/jobs/scheduled/old_keys_reminder.rb:7: warning: already initialized constant Jobs::OldKeysReminder::OLD_CREDENTIALS_PERIOD
/discourse/app/jobs/scheduled/old_keys_reminder.rb:7: warning: previous definition of OLD_CREDENTIALS_PERIOD was here
```

app/jobs/scheduled/old_keys_reminder.rb

@@ -4,12 +4,13 @@ module Jobs
   class OldKeysReminder < ::Jobs::Scheduled
     every 1.month
 
-    OLD_CREDENTIALS_PERIOD = 2.years
+    OLD_CREDENTIALS_PERIOD ||= 2.years
 
     def execute(_args)
       return if SiteSetting.send_old_credential_reminder_days.to_i == 0
       return if message_exists?
       return if old_site_settings_keys.blank? && old_api_keys.blank?
+
       PostCreator.create!(
         Discourse.system_user,
         title: title,