Deprecate CachingOperationInvoker#apply

[snicoll]

Unless I am missing the context CachingOperationInvoker#apply isn't called anywhere and I fail to see why we'd open such contract for public use.

If that's correct, we should probably deprecate it.

[dreis2211]

Maybe because the constructor of CachingOperationInvoker isn't public? Was wondering about it when working on #18961 as well.

[wilkinsona]

We'll deprecate it in 2.3.

[dreis2211]

I'd be happy to contribute. Just deprecating it and not changing anything else then (e.g. by making the constructor public)?

[wilkinsona]

Other than in tests, I can't see anything that requires the class to be public. It's tempting to deprecate the whole thing with a view to making it package-private in 2.4. I wonder if Spring Cloud uses it for some reason, @spencergibb?

[snicoll]

I've deprecated the method with a note. @spencergibb if Spring Cloud is using it after all, please let us know.