Bump stripe/stripe-php from 17.2.0 to 17.6.0 in /backend

Bumps [stripe/stripe-php](https://github.com/stripe/stripe-php) from 17.2.0 to 17.6.0.
- [Release notes](https://github.com/stripe/stripe-php/releases)
- [Changelog](https://github.com/stripe/stripe-php/blob/master/CHANGELOG.md)
- [Commits](stripe/stripe-php@v17.2.0...v17.6.0)

---
updated-dependencies:
- dependency-name: stripe/stripe-php
  dependency-version: 17.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]