Replace property 'lastCheck' with 'latestChange'

Once we have automated updates for the providers file, we should replace the property lastCheck with latestChange and update that by the automation tools if they changed the corresponding entry in the providers file. We should add a note to https://providers.xmpp.net for the daily checks/updates of the provider lists and replace the Last check info on each provider details page with Latest change.

Furthermore, I suggest that we link Something changed? on the website to a new section within its FAQ page. That section should tell the admins to provide the information via the XEPs mentioned in the automatic updates column in the properties section. And anyone else should be told to ask the providers to do that.