Make java version configurable

[tomasbjerre]

Proposal

Would be nice if the java version was configurable. Wiremock Spring Boot needs 17, as Spring Boot 3 is now 17.

I don't mind making it configurable. I just like to know I'm not wasting time on a pattern that you don't like.

I would suggest continuing with how Shadow is now configurable and add more properties. Like I've don in my personal conventions.

References

I was able to set the java version using the legacy syntax like this:
https://github.com/wiremock/wiremock-spring-boot/blob/dd1186e495451aac468556fe37963d3f973ea323/build.gradle#L16