Migrate to Dokka plugin V2

[Egorand]

Noticed this in CI build logs:

warning: Dokka Gradle plugin V1 is deprecated
    
    Dokka Gradle plugin V1 is deprecated, and will be removed in Dokka version 2.1.0
    Please migrate to Dokka Gradle plugin V2. This will require updating your project.
    To learn about migrating read the migration guide https://kotl.in/dokka-gradle-migration
    
    To start migrating to Dokka Gradle plugin V2 add
        org.jetbrains.dokka.experimental.gradle.pluginMode=V2EnabledWithHelpers
    into your project's `gradle.properties` file.
    
    We would appreciate your feedback!
     - Please report any feedback or problems https://kotl.in/dokka-issues
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