build: Use id instead of app-id in Flatpak manifest

Even though I couldn't find any information about it,
`app-id` is marked as deprecated in the JSON schema of
Flatpak manifests, so it triggers a warning in my IDE.

Since `id` seems to be supported everywhere, let's just use it.

build-aux/org.gnome.Fractal.Devel.json

@@ -1,5 +1,5 @@
 {
-    "app-id": "org.gnome.Fractal.Devel",
+    "id": "org.gnome.Fractal.Devel",
     "runtime": "org.gnome.Platform",
     "runtime-version": "master",
     "sdk": "org.gnome.Sdk",