Hardcode fake release date in metainfo.xml

Without a release date, flatpak ignores the version number, but
autodetecting the release date (and carrying it into tarballs) might be
too much work, so just hardcode an obviously fake date for now.

data/org.gnome.Hamster.metainfo.xml.in

@@ -45,7 +45,8 @@
   ​<translation type="gettext">hamster</translation>
   <url type="homepage">https://github.com/projecthamster/hamster/wiki</url>
   <releases>
-    <release version="@VERSION@" />
+    <!-- Provide a (clearly) fake date, otherwise flatpak will ignore the entry -->
+    <release version="@VERSION@" date="1980-01-01" />
   </releases>
   <provides>
     <binary>hamster</binary>