Bumpped minimum vimeo/psalm version to 3.15

Author: PHLAK

composer.json

@@ -31,7 +31,7 @@
         "friendsofphp/php-cs-fixer": "^2.16.1",
         "phpunit/phpunit": "^9.1",
         "psy/psysh": "^0.10",
-        "vimeo/psalm": "^3.8"
+        "vimeo/psalm": "^3.15"
     },
     "autoload": {
         "psr-4": {