Update Psalm version to 6.15.1 in workflow

Author: msyk

.github/workflows/psalm.yml

@@ -22,7 +22,7 @@ jobs:
         run: composer update
 
       - name: Install Psalm
-        run: composer require --dev vimeo/psalm:*
+        run: composer require --dev vimeo/psalm:6.15.1
 
       - name: Run Psalm
         run: ./vendor/bin/psalm --no-cache