test: bump rector php version to 8.4 (#699)

Author: marmichalski

rector.php

@@ -7,7 +7,7 @@
 
 return RectorConfig::configure()
     ->withPaths([__DIR__.'/src', __DIR__.'/tests'])
-    ->withPhpVersion(PhpVersion::PHP_83)
+    ->withPhpVersion(PhpVersion::PHP_84)
     ->withPHPStanConfigs([__DIR__.'/phpstan-rector.neon'])
     ->withImportNames(importShortClasses: false)
     ->withComposerBased(phpunit: true)