Bump org.jetbrains.dokka from 1.6.21 to 1.8.20 (#416)

Bumps [org.jetbrains.dokka](https://github.com/Kotlin/dokka) from 1.6.21 to 1.8.20.
- [Release notes](https://github.com/Kotlin/dokka/releases)
- [Commits](Kotlin/dokka@v1.6.21...v1.8.20)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

stub/build.gradle.kts

@@ -3,7 +3,7 @@ import org.jetbrains.dokka.gradle.DokkaTask
 import java.net.URL
 
 plugins {
-    id("org.jetbrains.dokka") version "1.6.21"
+    id("org.jetbrains.dokka") version "1.8.20"
 }
 
 repositories {