Bump org.jetbrains.dokka from 1.5.31 to 1.9.20

Bumps [org.jetbrains.dokka](https://github.com/Kotlin/dokka) from 1.5.31 to 1.9.20.
- [Release notes](https://github.com/Kotlin/dokka/releases)
- [Commits](Kotlin/dokka@v1.5.31...v1.9.20)

---
updated-dependencies:
- dependency-name: org.jetbrains.dokka
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

gradle/libs.versions.toml

@@ -18,4 +18,4 @@ rxJava2 = "io.reactivex.rxjava2:rxjava:2.2.21"
 [plugins]
 artifactory = { id = "com.jfrog.artifactory", version = "4.31.0" }
 kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
-kotlin-dokka = { id = "org.jetbrains.dokka", version = "1.7.0" }
+kotlin-dokka = { id = "org.jetbrains.dokka", version = "1.9.20" }