checkstyle: fix broken build

The Maven build configuration of the Checkstyle project requires Java 21 or a higher version. However, the default OSS-Fuzz base image provides Java 17, resulting in a mismatch error regarding the Java version.

Author: xingyaner

projects/checkstyle/Dockerfile

@@ -18,7 +18,12 @@ RUN curl -L https://archive.apache.org/dist/maven/maven-3/3.6.3/binaries/apache-
   -o maven.zip && \
   unzip maven.zip -d $SRC/maven && \
   rm maven.zip
-ENV MVN $SRC/maven/apache-maven-3.6.3/bin/mvn
+ENV MVN=$SRC/maven/apache-maven-3.6.3/bin/mvn
+RUN apt-get update && apt-get install -y openjdk-21-jdk \
+    && update-alternatives --set java /usr/lib/jvm/java-21-openjdk-amd64/bin/java \
+    && update-alternatives --set javac /usr/lib/jvm/java-21-openjdk-amd64/bin/javac
+ENV JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
+ENV PATH="$JAVA_HOME/bin:$PATH"
 RUN git clone --depth 1 https://github.com/checkstyle/checkstyle checkstyle
 COPY *.sh *Fuzzer.java $SRC/
 WORKDIR $SRC/checkstyle

projects/checkstyle/build.sh

@@ -16,7 +16,7 @@
 ##########################################################################
 export TARGET_PACKAGE_PREFIX="com.puppycrawl.tools.checkstyle."
 
-MAVEN_ARGS="-Dmaven.test.skip=true -Djavac.src.version=15 -Djavac.target.version=15 --update-snapshots"
+MAVEN_ARGS="-Dmaven.test.skip=true -Djavac.src.version=21 -Djavac.target.version=21 --update-snapshots"
 $MVN clean package $MAVEN_ARGS
 
 BUILD_CLASSPATH=