Set up Java 21 test

[suztomo]

The Java 21 build failed in https://github.com/GoogleCloudPlatform/app-maven-plugin/actions/runs/6356547529/job/17266373809?pr=491

[INFO] --- maven-plugin-plugin:3.6.4:descriptor (default-descriptor) @ appengine-maven-plugin ---
Error:  

Some dependencies of Maven Plugins are expected to be in provided scope.
Please make sure that dependencies listed below declared in POM
have set '<scope>provided</scope>' as well.

The following dependencies are in wrong scope:
 * org.apache.maven:maven-plugin-api:jar:3.8.1:compile
 * org.apache.maven:maven-model:jar:3.8.1:compile
 * org.apache.maven:maven-artifact:jar:3.8.1:compile
 * org.apache.maven:maven-core:jar:3.8.1:compile
 * org.apache.maven:maven-settings:jar:3.8.1:compile
 * org.apache.maven:maven-settings-builder:jar:3.8.1:compile
 * org.apache.maven:maven-builder-support:jar:3.8.1:compile
 * org.apache.maven:maven-repository-metadata:jar:3.8.1:compile
 * org.apache.maven:maven-model-builder:jar:3.8.1:compile
 * org.apache.maven:maven-resolver-provider:jar:3.8.1:compile

installPlugin: [INFO] --- spotbugs-maven-plugin:4.2.3:spotbugs (spotbugs) @ appengine-maven-plugin ---
installPlugin: [INFO] Fork Value is true
installPlugin:      [java] The following errors occurred during analysis:
installPlugin:      [java]   Error scanning java/lang/Thread for referenced classes
installPlugin:      [java]     java.lang.IllegalArgumentException: Unsupported class file major version 65
installPlugin:      [java]       At org.objectweb.asm.ClassReader.<init>(ClassReader.java:196)
installPlugin:      [java]       At org.objectweb.asm.ClassReader.<init>(ClassReader.java:177)
installPlugin:      [java]       At org.objectweb.asm.ClassReader.<init>(ClassReader.java:163)
installPlugin:      [java]       At edu.umd.cs.findbugs.asm.FBClassReader.<init>(FBClassReader.java:35)
installPlugin:      [java]       At edu.umd.cs.findbugs.classfile.engine.asm.ClassReaderAnalysisEngine.analyze(ClassReaderAnalysisEngine.java:48)
installPlugin:      [java]       At edu.umd.cs.findbugs.classfile.engine.asm.ClassReaderAnalysisEngine.analyze(ClassReaderAnalysisEngine.java:34)
installPlugin:      [java]       At edu.umd.cs.findbugs.classfile.impl.AnalysisCache.getClassAnalysis(AnalysisCache.java:261)
installPlugin:      [java]       At edu.umd.cs.findbugs.classfile.engine.ClassInfoAnalysisEngine.analyze(ClassInfoAnalysisEngine.java:75)
installPlugin:      [java]       At edu.umd.cs.findbugs.classfile.engine.ClassInfoAnalysisEngine.analyze(ClassInfoAnalysisEngine.java:38)
installPlugin:      [java]       At edu.umd.cs.findbugs.classfile.impl.AnalysisCache.getClassAnalysis(Analysi
...