Warning due to duplicate MockResolver extensions

[nosan]

#42957

I hope I’ve understood this issue correctly, but if not, please feel free to close this PR.

[snicoll]

Thanks @nosan. We can't remove the class itself as it is public API. Rather, we should stop registering by default as we used to be. I did that in 606709a

[nosan]

Thanks @snicoll
I had the same feeling that I could not touch this public class. However, I eventually decided to drop it as there’s a minimal chance that it's being used outside of Spring Boot itself. If anyone is using it, they can switch to SpringMockResolver as an alternative.

Anyway, your solution is better and safer than mine. Thank you.