Update dokka to v1.6.21

| datasource | package                                               | from   | to     |
| ---------- | ----------------------------------------------------- | ------ | ------ |
| maven      | org.jetbrains.dokka:org.jetbrains.dokka.gradle.plugin | 1.6.10 | 1.6.21 |
| maven      | org.jetbrains.dokka:dokka-core                        | 1.6.10 | 1.6.21 |

Author: renovate-bot

libs.versions.toml

@@ -4,7 +4,7 @@ arrow = "1.0.1"
 arrowGradleConfig = "0.10.3-alpha.1"
 coroutines = "1.6.1"
 kotlinxSerialization = "1.6.21"
-dokka = "1.6.10"
+dokka = "1.6.21"
 kotlinxSerializationJson = "1.3.3"
 ktor = "1.6.8"
 logbackClassic = "1.2.11"