Summary

• Moves maven.consumer.pom.flatten=true from pom.xml <properties> to .mvn/maven.properties
• The property is a Maven configuration setting, not a project property — it was silently ignored when placed in the POM
• With this fix, child module consumer POMs are fully self-contained 4.0.0 POMs with no <parent> reference to jline-parent (which uses modelVersion 4.1.0)
• This fixes Maven 3 and Gradle compatibility

Before (consumer POM still had parent reference):

<modelVersion>4.0.0</modelVersion>
<parent>
  <groupId>org.jline</groupId>
  <artifactId>jline-parent</artifactId>
  <version>4.0.1</version>
</parent>

After (self-contained consumer POM):

<modelVersion>4.0.0</modelVersion>
<groupId>org.jline</groupId>
<artifactId>jline-terminal</artifactId>
<version>4.0.1</version>

Fixes #1688

Workaround for apache/maven#11772