Bump spatie/icalendar-generator from 2.9.2 to 3.0.0 in /backend (#555)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/composer.json

@@ -22,7 +22,7 @@
         "nette/php-generator": "^4.0",
         "php-open-source-saver/jwt-auth": "^2.1",
         "sentry/sentry-laravel": "^4.13",
-        "spatie/icalendar-generator": "^2.8",
+        "spatie/icalendar-generator": "^3.0",
         "spatie/laravel-data": "^4.11",
         "spatie/laravel-webhook-server": "^3.8",
         "stripe/stripe-php": "^17.0"