Update the runtime to 42 and use fallback-x11

Author: bbhtt

flatpak/com.github.Alcaro.Flips.json

@@ -1,13 +1,13 @@
 {
     "app-id" : "com.github.Alcaro.Flips",
     "runtime" : "org.gnome.Platform",
-    "runtime-version" : "3.38",
+    "runtime-version" : "42",
     "sdk" : "org.gnome.Sdk",
     "command" : "flips",
     "finish-args" : [
         /* X11 + XShm access */
         "--share=ipc",
-        "--socket=x11",
+        "--socket=fallback-x11",
         /* Wayland access */
         "--socket=wayland",
         /* OpenGL + DRI access */