I have a maven project using Java 17. The maven-compiler-pugin is set to <release>17</release>.
If I build this project with a newer JDK version (for example 21), will the produced Jar still be runnable with a JRE 17?
Short answer - yes.
The release property you're using should cover both the produced class version and the usage of API that is available on Java 17.
There's always a caveat that you may be implicitly relying on some undocumented internal behavior and/or bug fix present in Java 21 that won't be present in Java 17, but those should really be rare cases.