AppImage: make appdata standard compliant (#9929)

- license: use SPDX license identifier: https://spdx.org/licenses/
- content rating: add OARS content rating: https://hughsie.github.io/oars/generate.html (required by flathub: https://github.com/flathub/flathub/wiki/AppData-Guidelines#oars-information)

platform/appimage/koreader.appdata.xml

@@ -3,7 +3,7 @@
   <id>org.koreader.KOReader</id>
   <launchable type="desktop-id">org.koreader.KOReader.desktop</launchable>
   <metadata_license>CC0-1.0</metadata_license>
-  <project_license>AGPL</project_license>
+  <project_license>AGPL-3.0-only</project_license>
   <name>KOReader</name>
   <summary>A document viewer for DjVu, PDF, EPUB and more</summary>
   <description>
@@ -24,4 +24,5 @@
     <category>Document viewers</category>
   </categories>
   <update_contact></update_contact>
+  <content_rating type="oars-1.1" />
 </component>