error: release version 21 not supported

Run mvn -v and make sure "Java Version" is 21, and not 17.
You may have to set JAVA_HOME to be pointing to the correct exe
If you're on cheesegrater, run:
export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-21.0.2.0.13-1.fc39.x86_64