Flatpak: switch to master branch of the runtime

.gitlab-ci.yml

@@ -7,7 +7,7 @@ variables:
     BUNDLE: "org.gnome.Podcasts.${CI_COMMIT_SHA}.flatpak"
 
 flatpak:
-    image: "registry.gitlab.gnome.org/gnome/gnome-runtime-images/rust_bundle:3.28"
+    image: "registry.gitlab.gnome.org/gnome/gnome-runtime-images/rust_bundle:master"
     stage: "test"
     variables:
         MANIFEST_PATH: "org.gnome.Podcasts.json"

org.gnome.Podcasts.json

@@ -1,7 +1,7 @@
 {
     "app-id" : "org.gnome.Podcasts",
     "runtime" : "org.gnome.Platform",
-    "runtime-version" : "3.28",
+    "runtime-version" : "master",
     "sdk" : "org.gnome.Sdk",
     "sdk-extensions" : [
         "org.freedesktop.Sdk.Extension.rust-stable"