Bump minimum PHP version to 8.1 & misc maintenance (#1521)

* Bump minimum PHP version to 8.1

Laravel 10 doesn't support 8.0 anyway

* Remove phpunit 9

* gha: remove unused matrix dimension

* gha: bump runner os

* tests: remove outdated version checks

composer.json

@@ -20,7 +20,7 @@
         }
     ],
     "require": {
-        "php": "^8.0",
+        "php": "^8.1",
         "ext-json": "*",
         "barryvdh/reflection-docblock": "^2.1.1",
         "composer/class-map-generator": "^1.0",
@@ -38,7 +38,7 @@
         "illuminate/view": "^9 || ^10 || ^11",
         "mockery/mockery": "^1.4",
         "orchestra/testbench": "^8 || ^9",
-        "phpunit/phpunit": "^9 || ^10.5",
+        "phpunit/phpunit": "^10.5",
         "spatie/phpunit-snapshot-assertions": "^4 || ^5",
         "vimeo/psalm": "^5.4"
     },