Bump testbench version so PHP8 stops complaining

.phpunit.result.cache

@@ -0,0 +1 @@
+C:37:"PHPUnit\Runner\DefaultTestResultCache":771:{a:2:{s:7:"defects";a:1:{s:67:"Tests\SignedUrlTest::a_signed_request_will_log_user_in_and_redirect";i:4;}s:5:"times";a:9:{s:56:"Tests\SignedUrlTest::can_create_default_signed_login_url";d:0.144;s:67:"Tests\SignedUrlTest::a_signed_request_will_log_user_in_and_redirect";d:0.023;s:61:"Tests\SignedUrlTest::an_unsigned_request_will_not_log_user_in";d:0.011;s:70:"Tests\SignedUrlTest::an_invalid_signature_request_will_not_log_user_in";d:0.018;s:59:"Tests\SignedUrlTest::allows_override_of_post_login_redirect";d:0.013;s:50:"Tests\SignedUrlTest::allows_alternative_auth_model";d:0.015;s:60:"Tests\SignedUrlTest::an_expired_request_will_not_log_user_in";d:1.019;s:41:"Tests\Unit\UserClassTest::make_from_class";d:0.013;s:40:"Tests\Unit\UserClassTest::make_from_slug";d:0.011;}}}

composer.json

@@ -16,7 +16,7 @@
  "illuminate/support": "^6.0|^7.0|^8.0"
  },
  "require-dev": {
- "orchestra/testbench": "^4.6"
+ "orchestra/testbench": "^6.0"
  },
  "autoload": {
  "psr-4": {