This commit removes
`RestTemplateBuilder.requestFactory(ClientHttpRequestFactory factory)`
because it can be misleading. This builder class is meant to be
immutable, but calling that method and then timeout related ones will
affect the `ClientHttpRequestFactory` instance.

Instead, this method is replaced with a
`Supplier<ClientHttpRequestFactory>` that is called every time a
`RestTemplate` is being built.

That approach may reduce the reusability of request factories, but it is
much more consistent.

Closes gh-11255