Add link to existing bug report

Author: sbrannen

spring-test/src/test/java/org/springframework/test/context/bean/override/mockito/MockitoBeanNestedTests.java

@@ -40,6 +40,7 @@
 @ExtendWith(SpringExtension.class)
 // TODO Remove @ContextConfiguration declaration.
 // @ContextConfiguration is currently required due to a bug in the TestContext framework.
+// See https://github.com/spring-projects/spring-framework/issues/31456
 @ContextConfiguration
 class MockitoBeanNestedTests {