Revert "Updated runtime to 6.8"

This reverts commit 9a114e4.

Author: XargonWan

net.rpcs3.RPCS3.yaml

@@ -1,6 +1,6 @@
 id: net.rpcs3.RPCS3
 runtime: org.kde.Platform
-runtime-version: '6.8'
+runtime-version: '6.7'
 sdk: org.kde.Sdk
 sdk-extensions:
   - org.freedesktop.Sdk.Extension.llvm18