Bump kotlin_version from 1.4.10 to 1.4.20

Bumps `kotlin_version` from 1.4.10 to 1.4.20.

Updates `kotlin-gradle-plugin` from 1.4.10 to 1.4.20

Updates `kotlin-stdlib-jdk8` from 1.4.10 to 1.4.20
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v1.4.20/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/compare/v1.4.10...v1.4.20

)

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