Migrate to Dokka plugin V2 (#2101)

[IRus]

• docs/changelog.md has been updated if applicable.
  • Changes not visible to library consumers, such as build script, documentation, or test code updates, don't need to
    be added to the changelog.
• CLA signed.

[IRus]

Plugin message after adding org.jetbrains.dokka.experimental.gradle.pluginMode=V2EnabledWithHelpers:

warning: Dokka Gradle plugin V2 migration helpers are enabled
    
    Thank you for migrating to Dokka Gradle plugin V2!
    Migration is in progress, and helpers have been enabled.
    To learn about migrating read the migration guide https://kotl.in/dokka-gradle-migration
    
    Once you have finished migrating disable the migration helpers by adding
        org.jetbrains.dokka.experimental.gradle.pluginMode=V2Enabled
    to your project's `gradle.properties` file.
    
    We would appreciate your feedback!
     - Please report any feedback or problems https://kotl.in/dokka-issues
     - Chat with the community visit #dokka in https://kotlinlang.slack.com/ (To sign up visit https://kotl.in/slack)
    

After migration to v2 org.jetbrains.dokka.experimental.gradle.pluginMode=V2Enabled:

    Thank you for enabling Dokka Gradle plugin V2!
    To learn about migrating read the migration guide https://kotl.in/dokka-gradle-migration
    
    We would appreciate your feedback!
     - Please report any feedback or problems https://kotl.in/dokka-issues
     - Chat with the community visit #dokka in https://kotlinlang.slack.com/ (To sign up visit https://kotl.in/slack)
    
    You can suppress this message by adding
        org.jetbrains.dokka.experimental.gradle.pluginMode.noWarn=true
    to your project's `gradle.properties` file.

And finally after adding org.jetbrains.dokka.experimental.gradle.pluginMode.noWarn=true clean logs

[Egorand]

Awesome, thank you!

[IRus]

I generated the documentation using the old dokkaHtml task and plugin, as well as the new task. I compared them visually and didn’t find any differences.