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

@@ -262,7 +262,7 @@
         <plugin>
           <groupId>org.jetbrains.dokka</groupId>
           <artifactId>dokka-maven-plugin</artifactId>
-          <version>1.6.20</version>
+          <version>1.6.21</version>
           <executions>
             <execution>
               <id>attach-javadocs</id>