Fix Apache license header in mixins.md and maven.config compatibility

- Add proper Apache Software Foundation license header to mixins.md
- Fix maven.config to work with Maven 3 by removing comment syntax
- Ensure full build passes with proper UTF-8 locale settings

Author: gnodet

.mvn/maven.config

@@ -1,2 +1 @@
-# A hack to pass on this property for Maven 3 as well; Maven 4 supports this property out of the box
 -DsessionRootDirectory=${session.rootDirectory}

src/site/markdown/mixins.md

@@ -1,3 +1,21 @@
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
 # Maven Mixins
 
 Maven Mixins provide a powerful mechanism for sharing common POM configuration across multiple projects without the limitations of traditional inheritance. Mixins allow you to compose your project configuration from multiple sources, promoting better code reuse and maintainability.