java: error: release version 21 not supported

This error occurs when there is a discrepancy between the Java compiler (JDK) version and the Maven file version. Java compiler version must be equal to or higher than the Maven version to avoid this error. One example that you might encounter this error would be when you create a project without checking the Java and packaging version as shown below

If you see the error below when running the project

Go to file -> project structure

And update the SDK version

Or open the pom.xml file and downgrade the parent and property version

And update the XML file by clicking the little icon on the top right (Ctrl + Shift + O)

Hope this helps