chore: fix PHP version for Psalm to 8.1

PHP 7.4 causes UndefinedFunction errors.

Error: src/Auth.php:54:15: MissingDependency: CodeIgniter\Shield\Models\UserModel depends on class or interface codeigniter\basemodel that does not exist (see https://psalm.dev/157)

Author: kenjis

psalm.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0"?>
 <psalm
+    phpVersion="8.1"
     errorLevel="7"
     resolveFromConfigFile="true"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"