SpringBoot Replaced all usages of Function<String, String> with UnaryOperator<String>

Continuing the work from 38390 to maintain consistency across the codebase

Comment From: abdullah-jaffer

Thank you, I have reverted the changes that might cause backward compatibility issues

Comment From: philwebb

I agree, #38390 was worthwhile because the API was new. I don't think we should change existing ones.