SpringBoot Update the Gradle plugin documentation to recommend the maven-publish plugin over the maven plugin

The documentation doesn't make any recommendation at the moment, other than documenting the maven plugin before the maven-publish plugin. We should swap the order over and also explicitly recommend the maven-publish plugin to align with Gradle's own recommendations and deprecations.

Comment From: wilkinsona

Closing in favour of https://github.com/spring-projects/spring-boot/pull/25299.