Added missing BC break

Author: ondrejmirtes

UPGRADING.md

@@ -300,3 +300,4 @@ Instead of `PHPStanTestCase::createBroker()`, call `PHPStanTestCase::createRefle
   * Interface `GlobalConstantReflection` renamed to `ConstantReflection`
 * Renamed interfaces and classes from `*WithPhpDocs` to `Extended*`
 * `ClassPropertyNode::getNativeType()` return type changed from AST node to `Type|null
+* Class `PHPStan\Node\ClassMethod` (accessible from `ClassMethodsNode`) is no longer an AST node