librem5: update-dialog: Reduce the default width

This avoids the dialog being too wide by default.

src/gs-update-dialog.ui

@@ -3,7 +3,7 @@
   <requires lib="gtk+" version="3.10"/>
   <template class="GsUpdateDialog" parent="GtkDialog">
     <property name="modal">True</property>
-    <property name="default_width">600</property>
+    <property name="default_width">350</property>
     <property name="default_height">600</property>
     <property name="destroy_with_parent">True</property>
     <property name="type_hint">dialog</property>