Clarify supported Target Java Version

[vorburger]

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

[cstamas]

While Maven build does not reflect it (edit: it does, but in a weird way) this is what Toolbox says:
https://gist.github.com/cstamas/f0dc3e72025952f03879cd5f8d209ae3

[vorburger]

In #251 this project seems to converge to restricting itself to using only Java 8 APIs (but building on recent JDK), if I understood correctly.

[cstamas]

Discussed this with @sormuras and it seems least friction is just to leave all on Java 11, as is today. But, I think we should use LTS build tooling, so apply similar pattern like we do in Maven or Maveniverse land:

• build time: LTS tools (Java 21, Maven 3.9.x)
• run time (bytecode level): Java 11 as today -- but probably we should test up to latest Java versions in a matrix?