Improve CI (Github Actions) to build against different Kotlin core versions (1.4 - 1.7)

[cowtowncoder]

(note: follow up for #556)

Currently the regular build as well as build on Continuous Integration (implemented using Github Actions) simply use the define version.kotlin dependency for everything.
But this won't help verifying whether build and tests would also work against other (older or newer) Kotlin core versions.

It seems like it is rather simple to override Kotlin dependency with

./mvnw -Dversion.kotlin=1.3.0 clean install test

(and so on).

So let's improve CI by adding alternate jobs or steps with different configurations.

[cowtowncoder]

Added to 2.13 branch, and it looks like Kotlin 1.5 and 1.6 pass, but 1.4 and 1.7 fail.

/cc @Spikhalskiy