Bump KMP to stable version 1.9.21

[buenaflor]

Description

It's worth considering bumping to the first stable version 1.9.20, if not now, then in the near future when most projects will have adopted 1.9.20.

Update:
This should be updated to 1.9.21 as 1.9.20 contains klib errors that affect all KMP libraries: https://github.com/JetBrains/kotlin/releases/tag/v1.9.21