Update Kotlin to 1.6.21
Currently, we have packaged Kotlin version 1.3.31. kotlin 1.7 is the first release to require gradle newer than 6.4, so 1.6.21 should be the next aim. Alternatively 1.3.72 is the earliest leap for latest gradle 7.3.
This task will be easier when build system patches that convert Gradle+Kotlin into Gradle+Groovy are dropped as described in issue #15 (closed). However, with some effort, it may be possible to upgrade to newer version (if not the latest) before that.
