Update stripe/stripe-php requirement from ^10.2 to ^12.0 in /backend

Updates the requirements on [stripe/stripe-php](https://github.com/stripe/stripe-php) to permit the latest version.
- [Release notes](https://github.com/stripe/stripe-php/releases)
- [Changelog](https://github.com/stripe/stripe-php/blob/master/CHANGELOG.md)
- [Commits](stripe/stripe-php@v10.2.0...v12.0.0)

---
updated-dependencies:
- dependency-name: stripe/stripe-php
  dependency-type: direct:production
...

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

backend/composer.json

@@ -17,7 +17,7 @@
         "jane-php/jane-php": "^4 <4.3",
         "miloschuman/yii2-highcharts-widget": "^10.0",
         "kartik-v/yii2-tabs-x": "@dev",
-        "stripe/stripe-php": "^10.2",
+        "stripe/stripe-php": "^12.0",
         "sleifer/yii2-autocomplete-ajax": "dev-master",
         "yiisoft/yii2-jui": "^2.0",
         "cebe/markdown": "^1.2",