Switch to newer flatpak runtime

Apparently the bug https://bugreports.qt.io/browse/QTBUG-99545 shouldn't
happen in the runtime anymore.

io.github.NhekoReborn.Nheko.yaml

@@ -1,9 +1,7 @@
 id: io.github.NhekoReborn.Nheko
 command: nheko
 runtime: org.kde.Platform
-#runtime-version: '5.15-21.08'
+runtime-version: '5.15-21.08'
-# use old runtime, newer one crashes on arm!
-runtime-version: '5.15'
 sdk: org.kde.Sdk
 rename-icon: nheko
 rename-desktop-file: nheko.desktop