Bump kotlin from 1.5.31 to 2.1.20 in /plugin

Bumps `kotlin` from 1.5.31 to 2.1.20.

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 1.5.31 to 2.1.20
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.5.31...v2.1.20)

Updates `org.jetbrains.kotlin:kotlin-reflect` from 1.5.31 to 2.1.20
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.5.31...v2.1.20)

Updates `org.jetbrains.kotlin:kotlin-script-runtime` from 1.5.31 to 2.1.20
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.5.31...v2.1.20)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-version: 2.1.20
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: org.jetbrains.kotlin:kotlin-reflect
  dependency-version: 2.1.20
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: org.jetbrains.kotlin:kotlin-script-runtime
  dependency-version: 2.1.20
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

plugin/gradle/libs.versions.toml

@@ -1,5 +1,5 @@
 [versions]
-kotlin = "1.5.31"
+kotlin = "2.1.20"
 ktlint = "0.47.1" # last version prior to API changes. we can increase this more once we drop support for the old API.
 androidPlugin = "4.1.0"
 semver = "1.1.1"