bugfix: fix extensions function name

[mogmet]

The function names used in extension have been changed.
In this PR, the function names have been modified to follow the changes.

https://github.com/stripe/stripe-firebase-extensions/blob/b2d6f244b378e117630a4adbb50aabf21961b525/firestore-stripe-payments/CHANGELOG.md#version-0115---2021-08-26

[mogmet]

@thorsten-stripe
Can I have this PR reviewed?

[mogmet]

@thorsten-stripe @cjavilla-stripe
Can I have this PR reviewed?

[labnol]

If you are getting any CORS errors in the Stripe Firebase app, it is due to this incorrect Firebase function name. You can make the change manually in your app.js file until Stripe devs merge this PR.

Before

    const functionRef = firebase
      .app()
      .functions(functionLocation)
      .httpsCallable('ext-firestore-stripe-subscriptions-createPortalLink');
    const { data } = await functionRef({ returnUrl: window.location.origin });
    window.location.assign(data.url);
  });

After

    const functionRef = firebase
      .app()
      .functions(functionLocation)
      .httpsCallable('ext-firestore-stripe-payments-createPortalLink');
    const { data } = await functionRef({ returnUrl: window.location.origin });
    window.location.assign(data.url);
  });