Previously, we relied on Mockito's internals to bypass any Spring AOP
proxy during verification of a spy. Thanks to a new API in Mockito,
we can replace the use of Mockito's internals with a
VerificationStartedListener. This listener changes Mockito's view of
the mock to be the ultimate target of the AOP proxy, i.e. to be the
actual Mockito-created spy, allowing Mockito's verification of the
spy to proceed successfully.

This above-described change will mean that we require a very
up-to-date version of Mockito so the tests that verify our
compatibility with 2.5 have been removed as we will no longer support
it when using @MockBean or @SpyBean.

Lastly, two tests have been updated to replace their usage of the
internal MockUtil class with the equivalent public API calls.

Closes gh-10352