Previously Java versions strings were 1.x, so the plugin-pom hardcodes a java.level property of "8" like so:

<source>1.${java.level}</source>

See: https://github.com/jenkinsci/plugin-pom/blob/master/pom.xml#L604

https://blog.codefx.org/java/java-9-migration-guide/#New-Version-Strings

But that won't work if we need to use a version of 9+, where the version string is now just "9" etc

In general we need multi-release JAR support here because if we change the way the java.level is handled then we don't inadvertently break Java 8 or earlier which expect the 1.x format.