Previously, if a user's configuration class provided a custom
Validator bean, that configuration class would be initialized very
early so that the Validator could be used to create the
auto-configured MethodValidationPostProcessor. This early
initialization could problems as it may prevent any of the
configuration class's dependencies from being post-processed.

This commit updates the injection of the Validator bean to be lazy,
thereby preventing the creation of the auto-configured
MethodValidationPostProcessor from triggering early initialization.

Closes gh-9416