Update Java to next LTE version (21)

[eladrion]

Feature Description

Premier Support for Java 17 is only guaranteed until September 2026, so we should switch to Java 21, which is also LTS. The mvnbuild.yml and pom.xml need to be updated accordingly. But then, we should also update some standard library use:

The constructor URL from java.net.URL is deprecated beginning in Java 20. By switching to java.net.URI and then instantiating an URI with URI ourURL = new URI("our_url"), we can get the URL by using ourURL.toURL().