Increase JDK baseline to JDK 8 for `java-uuid-generator` 5.0

[cowtowncoder]

It has been a while since I increased the JDK baseline and I think it would be reasonable to finally move to Java 8.
This would make it a little bit easier to maintain things (for example, able to build on JDK 17 which does not support Java 6 target).

This would be doable for version 4.1: I don't think a major version upgrade is warranted, although I am open to being convinced otherwise.

Feel free to add comments, especially if you have anything AGAINST proposal.

[anbusampath]

4.1 with Java 8.

[christophercurrie]

Arguing for 5.0: my rule of thumb for major version upgrades is when I answer "no" to the question: "can an existing system with a dependency on the previous version upgrade to the next version with no changes?" Which would not be the case for any Java 6 systems.

[cowtowncoder]

Thank you @anbusampath and @christophercurrie . Also received quite a bit of feedback via Twitter and that's so far heavily skewing towards 5.0. With history of JUG, I think that actually makes sense so my current thinking is to go with 5.0(.0).

[cowtowncoder]

Another reason to not do it yet for 4.1: there are a few features I am implementing, so can just release those as 4.1.
And then go Java 8 for 5.0 (and create 4.1 maintenance branch for possible (but unlikely :) ) patches).