fix: Move consumer POM flattening config to .mvn/maven.properties

The maven.consumer.pom.flatten property must be set in
.mvn/maven.properties (a Maven configuration property), not in
pom.xml <properties> (a project property). When set in the POM, it
was ignored and child consumer POMs still retained a <parent>
reference to jline-parent (which uses modelVersion 4.1.0), breaking
Maven 3 and Gradle consumers.

Workaround for apache/maven#11772

Author: gnodet

.mvn/maven.properties

@@ -0,0 +1 @@
+maven.consumer.pom.flatten=true

pom.xml

@@ -76,7 +76,6 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.build.outputTimestamp>${nisse.jgit.dateIso8601}</project.build.outputTimestamp>
-        <maven.consumer.pom.flatten>true</maven.consumer.pom.flatten>
 
         <java.build.version>22</java.build.version>
         <java.release.version>11</java.release.version>