[15.x] Adds Laravel 11 support

[nunomaduro]

This pull request adds Laravel 11 support. Please note the upgrade guide, we've added the following lines:

### Migration Changes

Cashier 15.0 no longer automatically loads migrations from its own migrations directory, so be sure to run the following command to publish Cashier's migrations to your application:

php artisan vendor:publish --tag=cashier-migrations