Require Maven 3.9.0 to build

minimalMavenBuildVersion goes from 3.6.3 to 3.9.0, so the
enforce-maven-version rule now demands 3.9.0 for every build, not only
for releases.

This is build-side only. plexus is a POM parent, it declares no
<prerequisites>, and requireMavenVersion is checked when a project is
built, never when its artifacts are consumed - so the minimum Maven for
anyone depending on a plexus component is unchanged. What changes is
that a contributor on Maven 3.6.x can no longer build.

The plexus-release profile already forced 3.9.0, so releases were built
with 3.9.0 while everything else was allowed to build with 3.6.3. That
override is now redundant and is removed; the two paths agree.

Generated-by: Claude Opus 5 (1M context)

Author: slachiewicz

pom.xml

@@ -337,7 +337,7 @@ limitations under the License.
 
   <properties>
     <javaVersion>8</javaVersion>
-    <minimalMavenBuildVersion>3.6.3</minimalMavenBuildVersion>
+    <minimalMavenBuildVersion>3.9.0</minimalMavenBuildVersion>
     <minimalJavaBuildVersion>${javaVersion}</minimalJavaBuildVersion>
     <maven.compiler.source>${javaVersion}</maven.compiler.source>
     <maven.compiler.target>${javaVersion}</maven.compiler.target>
@@ -901,7 +901,6 @@ limitations under the License.
     <profile>
       <id>plexus-release</id>
       <properties>
-        <minimalMavenBuildVersion>3.9.0</minimalMavenBuildVersion>
         <!-- use njord during release -->
         <njord.enabled>true</njord.enabled>
       </properties>