flatpak: Use 3.36 Sdk instead of master

The master Sdk doesn't have the rust-stable extension so we should use
3.36 until master has this or until we find another way to provide the
rust compiler to the Sdk.

flatpak/org.gnome.Fractal.json

@@ -1,7 +1,7 @@
 {
     "app-id" : "org.gnome.FractalDevel",
     "runtime" : "org.gnome.Platform",
-    "runtime-version" : "master",
+    "runtime-version" : "3.36",
     "sdk" : "org.gnome.Sdk",
     "sdk-extensions" : [
         "org.freedesktop.Sdk.Extension.rust-stable"