I am trying to deploy my project, a Spring Boot app, having added NIXPACKS_JDK_VERSION = 21 so that I can use java 21 instead of 17, but it still does not work. What I am doing wrong? I still get his message

Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:compile (default-compile) on project content-calendar: Fatal error compiling: error: release version 21 not supported -> [Help 1]

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