Bump stripe/stripe-php from 15.5.0 to 15.6.0

Bumps [stripe/stripe-php](https://github.com/stripe/stripe-php) from 15.5.0 to 15.6.0.
- [Release notes](https://github.com/stripe/stripe-php/releases)
- [Changelog](https://github.com/stripe/stripe-php/blob/master/CHANGELOG.md)
- [Commits](stripe/stripe-php@v15.5.0...v15.6.0)

---
updated-dependencies:
- dependency-name: stripe/stripe-php
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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