Add deprecation for method signature

Author: jderusse

CHANGELOG.md

@@ -13,7 +13,8 @@ CHANGELOG
  * Deprecated booting the kernel before running `WebTestCase::createClient()`
  * Deprecated `routing.loader.service`, use `routing.loader.container` instead.
  * Not tagging service route loaders with `routing.route_loader` has been deprecated.
-
+ * Overriding the methods `KernelTestCase::tearDown()` and `WebTestCase::tearDown()` without the `void` return-type is deprecated.
+ 
 4.3.0
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