Version 1.6.4

Mention Kotlin 1.6.21 in the README

Merge pull request Kotlin#3304 from Kotlin/version-1.6.2

Version 1.6.2

Merge pull request Kotlin#3232 from Kotlin/version-1.6.1

Version 1.6.1

Merge pull request Kotlin#3105 from Kotlin/version-1.6.0

Version 1.6.0

Version 1.6.0-RC3

Version 1.6.0-RC2

Merge pull request Kotlin#3035 from Kotlin/version-1.6.0-RC

Version 1.6.0-RC

Merge pull request Kotlin#2904 from Kotlin/version-1.5.2

Version 1.5.2

Merge pull request Kotlin#2812 from Kotlin/version-1.5.1

Version 1.5.1