It looks like the jar uploaded to jitpack requires java 16 or includes a library that requires java 16:
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-shade-plugin:3.2.4:shade (default-cli) on project ServerSelectorX: Error creating shaded jar: Problem shading JAR /home/robin/.m2/repository/com/github/TheSilentPro/HeadDB/4.2.0/HeadDB-4.2.0.jar entry net/wesjd/anvilgui/version/Wrapper1_17_R1.class: java.lang.IllegalArgumentException: Unsupported class file major version 60 -> [Help 1]
It looks like the jar uploaded to jitpack requires java 16 or includes a library that requires java 16:
In pom.xml compilation target is set to java 9: https://github.com/TheSilentPro/HeadDB/blob/417036e5186c3d3cf36146c32a283881f03fb0b2/pom.xml#L90-L91