Exclude NSRT test for bug-11894 from PHP 7.4

The test uses `mixed` type hint which requires PHP 8.0+.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-11894.php

@@ -1,4 +1,6 @@
-<?php declare(strict_types = 1);
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
 
 namespace Bug11894Nsrt;