Mark Reflection(Method|Property)#setAccessible() as pure starting from PHP 8.1 onwards

This will highlight unused code.

Ref: php/php-src#5412
Ref: https://wiki.php.net/rfc/make-reflection-setaccessible-no-op
Ref: php/php-src#5411

Example https://3v4l.org/PNeeZ

```php
<?php

class Foo {
    private $bar = 'baz';
    private function taz() { return 'waz'; }
}

//var_dump((new ReflectionProperty(Foo::class, 'bar'))->getValue(new Foo));
//var_dump((new ReflectionMethod(Foo::class, 'taz'))->invoke(new Foo));
```

Produces (starting from PHP 8.1):

```
string(3) "baz"
string(3) "waz"
```

Author: Ocramius

stubs/Php81.phpstub

@@ -31,6 +31,26 @@ namespace {
         public function isEnum(): bool {}
     }
 
+    class ReflectionProperty implements Reflector
+    {
+        /**
+         * Starting from PHP 8.1, this method is pure, and has no effect.
+         *
+         * @psalm-pure
+         */
+        public function setAccessible(bool $accessible): void {}
+    }
+
+    class ReflectionMethod extends ReflectionFunctionAbstract
+    {
+        /**
+         * Starting from PHP 8.1, this method is pure, and has no effect.
+         *
+         * @psalm-pure
+         */
+        public function setAccessible(bool $accessible): void {}
+    }
+
     /** @psalm-immutable */
     class ReflectionEnum extends ReflectionClass implements Reflector
     {