Upgrade PHPStan to version 2

 - phpstan/phpstan updated from 1.12.21 to 2.1.8 major
   See changes: phpstan/phpstan@1.12.21...2.1.8
   Release notes

   Had to remove redundant assertion. Not clear when false can actually occur on PHP ≥ 8.0 – on PHP 7.4, it can happen when passing an invalid target encoding but on PHP 8.0, it just throws a `ValueError`.

Author: jtojnar

composer.json

@@ -21,7 +21,7 @@
     },
     "require-dev": {
         "symfony/phpunit-bridge": "^6.2 || ^7.0",
-        "phpstan/phpstan": "^1.12",
+        "phpstan/phpstan": "^2.1",
         "friendsofphp/php-cs-fixer": "^3.4"
     },
     "suggest": {

src/MbTranscoder.php

@@ -72,9 +72,6 @@ public function transcode(string $string, $from = null, ?string $to = null): str
             $from
         );
 
-        // For PHPStan: We check the encoding is valid.
-        assert($result !== false);
-
         return $result;
     }