use modern spdx identifier

org.kde.kmouth.appdata.xml

@@ -2,7 +2,7 @@
 <component type="desktop-application">
   <id>org.kde.kmouth</id>
   <metadata_license>FSFAP</metadata_license>
-  <project_license>GPL-2.0+</project_license>
+  <project_license>GPL-2.0-or-later</project_license>
   <name>KMouth</name>
   <name xml:lang="ast">KMouth</name>
   <name xml:lang="ca">KMouth</name>