replace after annotation with attribute

PHPUnit 11 emits a deprecation warning when using annotations (@after), and they will not
work in PHPUnit 12.
Changing this means that the "after" functionality will not work in phpunit versions < 10,
but it also should not cause an error. Users directly calling "restoreEnvironmentVariables()"
would be fine, I think. See sebastianbergmann/phpunit#4502

src/EnvironmentVariables.php

@@ -12,10 +12,9 @@ trait EnvironmentVariables
     private $environmentVariables = [];
 
     /**
-     * @after
-     *
      * @return void
      */
+    #[\PHPUnit\Framework\Attributes\After]
     protected function restoreEnvironmentVariables()
     {
         foreach ($this->environmentVariables as $variable => $value) {