Plugin is a Maven 2.x plugin and mixes multiple Maven versions

If I execute:

$ mvn verify -Dmaven.plugin.validation=VERBOSE

on following POM:

<project ...>
    <modelVersion>4.0.0</modelVersion>

    <groupId>...</groupId>
    <artifactId>...</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.release>21</maven.compiler.release>
        <scala.major.version>3</scala.major.version>
        <scala.version>${scala.major.version}.3.3</scala.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala3-library_3</artifactId>
            <version>3.4.2</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>net.alchim31.maven</groupId>
                <artifactId>scala-maven-plugin</artifactId>
                <version>4.9.1</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>
                            <goal>testCompile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

I got:

[WARNING] 
[WARNING] Plugin [INTERNAL, EXTERNAL] validation issues were detected in following plugin(s)
[WARNING] 
[WARNING]  * net.alchim31.maven:scala-maven-plugin:4.9.1
[WARNING]   Plugin EXTERNAL issue(s):
[WARNING]    * Plugin is a Maven 2.x plugin, which will be not supported in Maven 4.x
[WARNING]    * Plugin mixes multiple Maven versions: [3.3.9, 2.2.1]
[WARNING] 
[WARNING] 
[WARNING] Fix reported issues by adjusting plugin configuration or by upgrading above listed plugins. If no upgrade available, please notify plugin maintainers about reported issues.
[WARNING] For more or less details, use 'maven.plugin.validation' property with one of the values (case insensitive): [NONE, INLINE, SUMMARY, BRIEF, VERBOSE]
[WARNING]

davidB / scala-maven-plugin

Plugin is a Maven 2.x plugin and mixes multiple Maven versions #793