Replace Mockito argument captors with assertArg

Fixes #34943

Comment From: pivotal-cla

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Comment From: pivotal-cla

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Comment From: mhalbritter

Thank you very much and congratulations on your first contribution :tada:!

Comment From: wilkinsona

I think there are some more places where we can use assertArg. I started polishing in https://github.com/wilkinsona/spring-boot/commit/33b8dfbf35ab196ea0d6a1d16405a63924e6a8c6 I stopped as I reached a place where https://github.com/mockito/mockito/issues/2982 would be useful. I think it'd be nice to move over everywhere we can without the enhancement though.