Improve property validation example to not use static runner class

[lnunno]

It would be nice if the Spring property validation example here.

Did not use a static runner class to demonstrate whether or not this is required. I have a StackOverflow question here that documents the issues I've been having.

[philwebb]

I've answered your stackoverflow question but we should also make some updates here. I think the sample should use a static configurationPropertiesValidator bean method and we should add some warnings to the documentation.