Add kotlin template for redis

[AleksandrSl]

Thank you for the awesome project. Want to add a little bit more Kotlin to it.

• New version (1.6.20) of kotlin compiler. I searched for the image on dockerhub, but there are no official images, I don't usually trust other users, and the current author didn't update their images after 1.4. I used alpine version to be smaller, 200mb vs >500mb for slim
• Kotlin template for redis

Maybe we could also remove 1.3.72 kotlin dockerfile? I would remove 1.4 also because it is 2 years old, and not so much used to be someone's preference I believe

[rohitpaulk]

Thanks so much for the PR @AleksandrSl! Thought I'd leave a quick comment here now acknowledging this, I'm aiming to review & deploy sometime this week.

[rohitpaulk]

We've been running into some issues with kotlin compilation times which I'm fixing in https://github.com/codecrafters-io/languages/pull/121, will get to this once that's done!