Update laravel/cashier requirement from ^12.5 to ^13.6

Updates the requirements on [laravel/cashier](https://github.com/laravel/cashier-stripe) to permit the latest version.
- [Release notes](https://github.com/laravel/cashier-stripe/releases)
- [Changelog](https://github.com/laravel/cashier-stripe/blob/13.x/CHANGELOG.md)
- [Commits](laravel/cashier-stripe@v12.5.0...v13.6.0)

---
updated-dependencies:
- dependency-name: laravel/cashier
  dependency-type: direct:development
...

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

Author: dependabot[bot]

composer.json

@@ -33,7 +33,7 @@
     "brianium/paratest": "^6.2",
     "cknow/laravel-money": "^6.1",
     "infection/infection": "~0.17",
-    "laravel/cashier": "^12.5",
+    "laravel/cashier": "^13.6",
     "nunomaduro/collision": "^5.4",
     "orchestra/testbench": "^6.4",
     "phpmetrics/phpmetrics": "^v2.7",