Add PHP 8 compatibility

```
vendor/bin/phpcs -p vendor/msp/adminrestriction --standard=vendor/phpcompatibility/php-compatibility/PHPCompatibility --runtime-set testVersion 8.0
...... 6 / 6 (100%)


Time: 152ms; Memory: 10MB

vendor/bin/phpcs -p vendor/msp/adminrestriction --standard=vendor/phpcompatibility/php-compatibility/PHPCompatibility --runtime-set testVersion 8.1
...... 6 / 6 (100%)


Time: 145ms; Memory: 10MB
```

composer.json

@@ -2,7 +2,7 @@
     "name": "msp/adminrestriction",
     "description": "Magento2 Admin IP based restriction - Member of MageSpecialist SecuritySuite",
     "require": {
-        "php": "^7.0|^7.1",
+        "php": "^7.0|^8.0.0|^8.1.0",
         "msp/security-suite-common": "^2.0",
         "magento/magento-composer-installer": "*"
     },
@@ -25,4 +25,4 @@
             "MSP\\AdminRestriction\\": ""
         }
     }
-}
+}