Clarify supported Target Java Version

IMHO it would be useful to clarify the supported target Java Version on the README - and then enforce that on CI.

In #214 @kevodwyer clarified @x-xoom that it's something more recent than Java 8.

But that still leaves the question... is it cool to use Java 23 only features in this project? 😄  If not, what's the maximum?

@ianopolous 