Handle forward compatibility & JVM target version

[jayasuryat]

Concerns:

• JVM target-version seems to be not pegged properly
• All of the modules are compiled with Kotlin 1.9.0 & making them not forward-compatible with, let say ~1.6.0
• The Kotlin-std-lib is getting bundled in into the artifacts

[jayasuryat]

Binaries are now compatible with Kotlin-1.5.0

[jayasuryat]

Turns out, just setting kotlinOptions.apiVersion & kotlinOptions.languageVersion to version-x will not make it (forward) compatible with version-x. And also Kotlin makes no such guarantees about forward-compatibility too.

(Taken from https://kotlinlang.org/docs/kotlin-evolution.html#evolving-the-binary-format)

Preferably (but we can't guarantee it), the binary format is mostly forwards compatible with the next feature release, but not later ones (in the cases when new features are not used, e.g. 1.3 can understand most binaries from 1.4, but not 1.5).

So, even tho if you set kotlinOptions.apiVersion & kotlinOptions.languageVersion to an appropriate version and don't use any new features added to Kotlin after that version, there is no guarantee that it would be compatible with that version (while compiling with a newer version).

---

With the current setup (compiling with Kotlin 1.9.0, and languageVersion set to 1.5.0), artifacts are compatible upto Kotlin 1.7.0, (came to this conclusion by testing my self, didn't find any source on this).

Which I feel is pretty decent for now.

[jayasuryat]

With the current situation, we can also relax kotlinOptions.apiVersion & kotlinOptions.languageVersion to Kotlin 1.7.0.