Add license to info block

[mnahkies]

Similar vein to https://github.com/stripe/openapi/issues/72 - there is now a LICENSE file in the repo which is great, but if it's not too much hassle it would be nice to include this in the specifications themselves. Eg:

info:
  contact:
    email: [email protected]
    name: Stripe Dev Platform Team
    url: 'https://stripe.com'
  description: >-
    The Stripe REST API. Please see https://stripe.com/docs/api for more
    details.
  license:
    name: MIT
    url: https://spdx.org/licenses/MIT
  termsOfService: 'https://stripe.com/us/terms/'
  title: Stripe API
  version: '2022-11-15'

ref: https://spec.openapis.org/oas/v3.0.3#info-object