Issue #3507536: Switch to conditional usage of PHPStan for upstream incompatibilities

Closes #3507536

phpstan-drupal-core-bc-layer.neon.php

@@ -11,10 +11,10 @@ use Drupal\Core\Recipe\Recipe;
 
 $includes = [];
 if (method_exists(Recipe::class, 'getExtra')) {
-  $includes[] = __DIR__ . '/phpstan-baseline-getExtras.neon';
+  $includes[] = 'phpstan-baseline-getExtras.neon';
 }
 else {
-  $includes[] = __DIR__ . '/phpstan-baseline-no-getExtras.neon';
+  $includes[] = 'phpstan-baseline-no-getExtras.neon';
 }
 
 $config = [];