When Spring Framework builds a `RestClient` from a `RestTemplate`, it
will use any `UriTemplateHandler` that has been set on the
`RestTemplate` if the provided `UriTemplateHandler` is also a
`UriBuilderFactory`. Prior to this commit, Spring Boot's
`RestTemplateBuilder#rootUri` set a `UriTemplateHandler` on the created
`RestTemplate`, but it was not a `UriBuilderFactory` so `RestClient`
would not consider it.
With this commit, `RestTemplateBuilder#rootUri` sets a
`UriTemplateHandler` that is also a `UriBuilderFactory` so that any
root URI that is set on the `RestTemplateBuilder` will be applied to a
`RestClient` also.
Fixes gh-39317
720e9cef16