[15.x] Always end trial when subscription is cancelled

[driesvints]

This change in behavior will cause subscriptions to always skip their trial once they're ended/cancelled. It's more in line on how Stripe works/behaves. Once a subscription is cancelled after this change it will not be active anymore when there's still an ongoing trial. This also better accommodates the expected behavior when cancelling subscriptions from the Stripe Dashboard "immediately" which was a long outstanding weird behavior when the subscription was still on trial.

I thought about this long and hard and I think this is the best choice going forward. I considered adding an option to opt-in to the old behavior but I believe it makes most sense that a subscription isn't active anymore after it's considered cancelled or ended.

Targetting the next major version because this is a breaking change.