Deprecate RootUriTemplateHandler in favor of DefaultUriBuilderFactory

[philwebb]

RootUriTemplateHandler was added in Spring Boot 1.4 before DefaultUriBuilderFactory existed. The logic to prepend a root URL now duplicates what DefaultUriBuilderFactory#initUriComponentsBuilder is capable of doing by merging two parsed UriComponent instances.

[philwebb]

This will also effect RootUriRequestExpectationManager.

[philwebb]

See #48337