Bump phpstan-drupal to 0.12.15

Adds recent releases to improve service return types

Author: mglaman

composer.json

@@ -13,7 +13,7 @@
         "php": "^7.2.5|^8.0",
         "composer/xdebug-handler": "^1.1 | ^2.0.1",
         "jean85/pretty-package-versions": "^1.5.0 || ^2.0.1",
-        "mglaman/phpstan-drupal": "^0.12.12",
+        "mglaman/phpstan-drupal": "^0.12.15",
         "nette/neon": "^3.1",
         "phpstan/phpstan-deprecation-rules": "^0.12.6",
         "symfony/console": "~3.2 || ~4.0",