Make Maven 3.9.x required on implementation projects

On 15.7, and current 16.0, there's

<maven.min.version>3.5.4</maven.min.version>

which is used as <requireMavenVersion> on the maven-enforcer-plugin.

This Maven version is enforced on the build of both the product code base as an implementation project.

The idea is to move the minimum to the latest minor, first patch, so Maven 3.9.0. See https://maven.apache.org/docs/history.html

Related: https://xmdocumentation.bloomreach.com/trails/getting-started/prerequisites.html