Move to java17 as default

[serpro69]

This is to gather details on moving to java17 as minimum requirement and start with the implementation.
The plan is to release this in 2.0.0.

Acceptance criteria:

• java17 is the minimal java version for kotlin-faker:2.0.0
• java8 is supported (via own artifact? kotlin-faker-jdk8?)
  • (optionally java11 should also be supported in a similar fashion?)
• kotlin 2.0.0 (if released by then) is the minimal kotlin version for kotlin-faker:2.0.0
  • backwards compatibility with 1.[789].x is unclear at this point

Some useful documentation from gradle:

• https://docs.gradle.org/current/userguide/feature_variants.html
• https://docs.gradle.org/current/userguide/toolchains.html

[serpro69]

OK, I'm not really sure this makes sense for a kotlin-based library. What benefits do we get from defaulting to java17 when the primary language is kotlin? I can't see any, at least until kotlin starts declaring "minimal supported java version".
At the same time, implementing this issue would require introduction of another artifact to make the lib compatible with java8.
Need to think more on it before breaking java8 compatibility.