Bump stripe/stripe-php from 13.8.0 to 13.9.0 in /_backend

Bumps [stripe/stripe-php](https://github.com/stripe/stripe-php) from 13.8.0 to 13.9.0.
- [Release notes](https://github.com/stripe/stripe-php/releases)
- [Changelog](https://github.com/stripe/stripe-php/blob/master/CHANGELOG.md)
- [Commits](stripe/stripe-php@v13.8.0...v13.9.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

_backend/composer.json

@@ -11,7 +11,7 @@
         }
     },
     "require": {
-        "stripe/stripe-php": "13.8.0",
+        "stripe/stripe-php": "13.9.0",
         "sentry/sdk": "^3.3.0",
         "erusev/parsedown-extra": "^0.8",
         "geoip2/geoip2": "^2.0",