desktop: update release date.

Author: Jehan

desktop/org.gimp.GIMP.appdata.xml.in.in

@@ -82,7 +82,7 @@
   </provides>
 
   <releases>
-    <release version="3.1.4" date="2025-08-14" type="development">
+    <release version="3.1.4" date="2025-08-30" type="development">
       <description>
         <p>
         Our second development release finally contains the initial implementation of the two new types of