Update to latest Kotlin and enable explicit API mode

[Maxr1998]

🔑 Use case

Since the public methods exposed by the classes already have an explicit visibility modifier, it might make sense to enforce this with the explicit API mode, introduced in Kotlin 1.4.

🔨 Proposal

For explicitApi() to be enabled in the Gradle build script, the Kotlin version needs to be at least 1.4.

🐝 Request for assignment

If necessary, I could implement this.

Note: this is actually a feature request, but it seems the label I added was wrong.