Prior to this commit, gh-48310 separated client and server message
converter configurations by switching from message converter instances
as beans in the application context, to server/client customizers that
are applied to the `HttpMessageConverters` instances while being built.
This change did not order the new ClientHttpMessageConvertersCustomizer
or ServerHttpMessageConvertersCustomizer, letting those being at the
"lowest precedence" default. As customizers, this means they are applied
last and custom instances cannot take over.
This commit ensures that such customizers provided by Spring Boot are
now ordered at "0" to let applications ones take over.
Fixes gh-48609