Switch Prowlarr back to master branch

Back to the time when Prowlarr was added to Ultra, it wasn't available as a stable (master) release, and thus has been deployed on develop. Now that it is officially available for a while now, it would be cool to switch back to the master branch. Technically speaking, current updates (on Ultra) would have to be blocked for some time, in order to wait for master to be ahead of the Ultra's frozen develop branch. Although this is a small change, I think this would be better stabilty-wise as a shared provider.