appstreamcli doesn't validate timestamp

[JakobDev]

There was a Bug report for the Flathub website, that the release date, was 52 years ago. I checked the Appstream file of the App which contains this lines:

<releases>
  <release version="1.0.3" timestamp="2021-11-15"/>
  <release version="1.0.2" timestamp="2021-06-06"/>
  <release version="1.0.1" timestamp="2020-08-09"/>
  <release version="1.0.0" timestamp="2020-08-09"/>
</releases>

This file accidentality uses timestamp instead of Date. I downloaded the file to see if appstreamcli find the error. The file pass the validation. I know, that Flathub currently uses appstream-glib, but Flatpak and Flathub working on a switch to libappstream, so it would be nice if you could add a check so these things can be blocked in the future.