Drop Kotlin 1.4 support from Kotlin module 2.15

[cowtowncoder]

As of Jackson 2.14, Kotlin module works (and is tested to work via CI) with kotlin-core versions 1.4 through 1.7 (1.8 is an open question). Default dependency is to kotlin-core 1.5.

But it seems that there is diminishing value in trying to keep compatibility with kotlin-core 1.4 and it has been suggested it would make sense to drop support. This issue is proposing doing that with Jackson-module-kotlin 2.15.0 release.

If we decide to proceed with this, work needed includes:

• Update README.md with changed state wrt compatibility
• Remove 1.4 from matrix build in .github/workflows/main.yml
• Add a note in https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.15 wrt Kotlin compatibility change

[cowtowncoder]

cc @k163377 @dinomite @viartemev

[k163377]

Memo:
I also intend to add the following check
#628 (comment)

[Spikhalskiy]

If Kotlin 1.4 support is dropped, it's better to set a language level or build against Kotlin std lib in a way that makes jackson-module-kotlin not binary compatible with Kotlin 1.4 at all.
Build-time failure is better than unpredictable misbehaving at runtime.
It can be achieved by building against Kotlin 1.6, but testing against >=1.5. Kotlin builds are binary compatible one minor version down. So Kotlin 1.5 will be supported by such a build, but not Kotlin 1.4.

[cowtowncoder]

@Spikhalskiy That sounds like a good idea. So the recently closed PR that was to build using 1.7 was close but could be modified to do this.

[cowtowncoder]

Although changes to README, CI are in, I hope we can consider additional changes to build, runtime checks for kotlin-core in use (to prevent users from accidentally attempting to use with pre-1.5 Kotlin).

[SimonaKempleG7]

This is a discussion about dropping support for Kotlin 1.4 in the Jackson module. It seems that there is agreement that dropping support would make sense, and the work needed to do this is outlined. Some contributors have also suggested ways to ensure that the module is not binary compatible with Kotlin 1.4 at all, which would be better than unpredictable misbehaving at runtime. The idea of building against Kotlin 1.6 and testing against 1.5 is suggested to achieve this goal.

[cowtowncoder]

Completed via #646: Jackson 2.15 version of this module will require Kotlin core of 1.5 or higher.