Prepare for the release of Dokka 2.0.0

[IgnatBeresnev]

Context

The release of Kotlin 2.0.0 is approaching, and it's expected that many people may want to try Kotlin 2.0 once it's out.

Even though Kotlin 2.0 should work perfectly well with Dokka 1.9.20, it makes sense to release Dokka 2.0.0 to avoid any confusion as to which version of Dokka works with which version of Kotlin, allowing users to bump both at roughly the same time.

TODO

1. Update the Dokka 2.0.0 milestone, make sure it has relevant issues.
2. Make sure that publishing to staging works in CI (there were problems with it around 1.9.20)
   • Required: Allow publishing to a pre-defined staging repository #3562
   • Required: Do not require signing artifacts when publishing locally #3561
3. Make sure that all issues blocking the release have been merged:
   • Required: Avoid failing Dokka generation in jvm+androidJvm projects #3572
4. Create the release branch