Update references for RestTemplateBuilder and Customizer

Starting from Spring Boot 4, the RestTemplateBuilder and
RestTemplateCustomizer classes have been moved from the
org.springframework.boot.web.client package to the
org.springframework.boot.restclient package. This commit updates
the javadoc links in the reference documentation to match

Signed-off-by: Antoine Rey <antoine.rey@free.fr>

See gh-48295

Author: arey

documentation/spring-boot-docs/src/docs/antora/modules/how-to/pages/http-clients.adoc

@@ -9,7 +9,7 @@ This section answers questions related to using them.
 [[howto.http-clients.rest-template-proxy-configuration]]
 == Configure RestTemplate to Use a Proxy
 
-As described in xref:reference:io/rest-client.adoc#io.rest-client.resttemplate.customization[RestTemplate Customization], you can use a javadoc:org.springframework.boot.web.client.RestTemplateCustomizer[] with javadoc:org.springframework.boot.web.client.RestTemplateBuilder[] to build a customized javadoc:org.springframework.web.client.RestTemplate[].
+As described in xref:reference:io/rest-client.adoc#io.rest-client.resttemplate.customization[RestTemplate Customization], you can use a javadoc:org.springframework.boot.restclient.RestTemplateCustomizer[] with javadoc:org.springframework.boot.restclient.RestTemplateBuilder[] to build a customized javadoc:org.springframework.web.client.RestTemplate[].
 This is the recommended approach for creating a javadoc:org.springframework.web.client.RestTemplate[] configured to use a proxy.
 
 The exact details of the proxy configuration depend on the underlying client request factory that is being used.