Raise Stripe's subscription fee to 0.7% on June 30, 2025

[frankie567]

Stripe is deprecating the cheapest Billing plan (subscriptions management). Hence, the fee raises from 0.5% to 0.7%. They give us a one year grace period where the current pricing is still applied, i.e. until June 30, 2025.

Remember to change this at that date so we echo that price hike.

https://github.com/polarsource/polar/blob/240e1d796e890c26e96d191eb0515ee61ec5744a/server/polar/transaction/fees/stripe/init.py#L41-L42