Bump dokka-maven-plugin from 1.6.20 to 1.6.21

Bumps [dokka-maven-plugin](https://github.com/Kotlin/dokka) from 1.6.20 to 1.6.21.
- [Release notes](https://github.com/Kotlin/dokka/releases)
- [Commits](https://github.com/Kotlin/dokka/commits)

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

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -40,7 +40,7 @@
  <junit.jupiter.version>5.8.2</junit.jupiter.version>
  <slf4j.version>1.7.36</slf4j.version>
  <log4j.version>2.13.3</log4j.version>
- <dokka.version>1.6.20</dokka.version>
+ <dokka.version>1.6.21</dokka.version>
 
  <!--
  Deactivate dokka by default, and make the build faster for all projects.