Remove CC-BY as main project license

Some of our build files and resource files are licensed as that, but not
our actual code. This leads to funny issues, that GNOME software
considers Nheko proprietary. To work around that, just put GPL there.

Fixes https://github.com/flathub/io.github.NhekoReborn.Nheko/issues/1

resources/nheko.appdata.xml

@@ -3,7 +3,7 @@
 <component type="desktop">
   <id>nheko.desktop</id>
   <metadata_license>CC0-1.0</metadata_license>
-  <project_license>GPL-3.0-or-later and CC-BY</project_license>
+  <project_license>GPL-3.0-or-later</project_license>
   <name>nheko</name>
   <summary>Desktop client for the Matrix protocol</summary>
   <description>