restTemplateBuilderConfigurer doesn't need to be @ConditionalOnMissingBean

[wilkinsona]

restTemplateBuilderConfigurer defined by RestTemplateAutoConfiguration is @ConditonalOnMissingBean but that condition doesn't appear to be necessary. RestTemplateBuilderConfigurer is a final class and is immutable from the perspective of its public API. You could define a custom configurer bean but it would be in its default state and would do nothing when asked to configure the RestTemplateBuilder. You could just define a RestTemplateBuilder bean directly instead:

@Bean
public RestTemplateBuilder restTemplateBuilder() {
	 return new RestTemplateBuilder();
}

We should remove @ConditonalOnMissingBean. In the unlikely event of someone having defined their own RestTemplateBuilderConfigurer this would be a breaking change as two RestTemplateBuilderConfigurer beans would then be defined. As such, we should remove the condition in 3.2 at the earliest and mention the change in the release notes.

[wilkinsona]

Closing in favor of #37746.