Update Kotlin to latest version

[ErikSchierboom]

The exercises are using a somewhat outdated version of Kotlin: 1.3.60, whereas the latest version is 1.5.21.

Updating to the latest version will allow students to use all the latest and greatest features.

[incendium]

This can be closed as #505 has resolved this issue.

[SleeplessByte]

Thanks @incendium for the ping.