Update Kotlin to 1.4
Relevant info:
https://kotlinlang.org/docs/reference/whatsnew14.html#migrating-to-kotlin-140
https://kotlinlang.org/docs/reference/migrating-multiplatform-project-to-14.html
https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/migration.md
https://github.com/Kotlin/dokka/blob/master/runners/gradle-plugin/MIGRATION.md
Edited by Incendium