Why does Maven warn me about encoding?

You haven’t set the encoding default property like this:

<project>
  ...
  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>
  ...
</project>

This approach is better than defining encoding manually for every plugin, cause all plugins having default values for encoding for example the maven-resources-plugin:

encoding:

The character encoding scheme to be applied when filtering resources.
Type: java.lang.String
Required: No
User Property: encoding
Default: ${project.build.sourceEncoding}

So this means you only need to define this property and the plugin will automatically use this encoding.