flatpak: Build Flatpak using GNOME 42

So the old meson in the 3.36 runtime doesn't choke on:
tests/meson.build:46:7: ERROR: Unknown method "project_source_root" in object.

.gitlab-ci.yml

@@ -136,7 +136,8 @@ test_unsupported_list:
 flatpak:
   stage: flatpak
   extends: .flatpak
-  image: registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:3.36
+  # From https://gitlab.gnome.org/GNOME/gnome-runtime-images/container_registry
+  image: registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:42
   variables:
     MANIFEST_PATH: "demo/org.freedesktop.libfprint.Demo.json"
     FLATPAK_MODULE: "libfprint"

demo/org.freedesktop.libfprint.Demo.json

@@ -1,7 +1,7 @@
 {
     "app-id": "org.freedesktop.libfprint.Demo",
     "runtime": "org.gnome.Platform",
-    "runtime-version": "3.36",
+    "runtime-version": "42",
     "sdk": "org.gnome.Sdk",
     "command": "gtk-libfprint-test",
     "finish-args": [