Require a released flowdrop_ui before the first tag

Problem

`composer.json` requires `drupal/flowdrop_ui: 2.x-dev` and adds a `repositories` entry for flowdrop_ui's git repository. A release that depends on a dev version cannot be installed on sites with `minimum-stability: stable`, and Composer ignores `repositories` in anything but the root package, so the entry only helps local development.

Proposal

Once flowdrop_ui ships a release with what this module needs, require that release (`^2.x.y`) and drop the `repositories` entry.

Expected

- `composer require drupal/flowdrop_ui_modeler` works on a stock Drupal site.
- CI still tests against flowdrop_ui's dev branch where useful.