Enable new GitLab CI infra for flatpak
The skrooge Nightly flatpak is no longer getting built at invent.kde.org nor published to the kdeapps flatpak remote. Per https://develop.kde.org/docs/packaging/flatpak/publishing/ , rename org.kde.skrooge.json to .flatpak-manifest.json and add the flatpak CI template to Skroog's GitLab CI file.
BUG: 481530