Streaming payments only support a single stream

[area]

NB Based on maint/pnpm.

Multiple streams in a single streaming payment are no longer desired on the frontend, so taking the opportunity to simplify the contracts. This will also allow accurate tracking of the number of pending payments, allowing the 'prevent uninstall if pending payments' feature.

I've taken advantage of finishUpgrade to make sure someone can't upgrade from v4 to v5, which we are declaring is impossible due to the storage layout changes.