$ xmllint --schema /tmp/maven-4.0.0.xsd parent/pom.xml --noout
parent/pom.xml:352: element configuration: Schemas validity error : Element '{http://maven.apache.org/POM/4.0.0}configuration', attribute 'combine.self': The attribute 'combine.self' is not allowed.
parent/pom.xml fails to validate

available values are <code>merge</code> (default) and <code>override</code>.

<plugin>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.10.1</version>
  <configuration>
    <release>11</release>
    <encoding>UTF-8</encoding>
    <useIncrementalCompilation>false</useIncrementalCompilation>
    <parameters>true</parameters>
    <showDeprecation>true</showDeprecation>
    <showWarnings>true</showWarnings>
    <source>11</source>
    <target>11</target>
    <testSource>11</testSource>
    <testTarget>11</testTarget>
    <compilerArgs>
      <arg>-Xlint:unchecked</arg>
    </compilerArgs>
  </configuration>
</plugin>