Update to AGP 9.1.0

[MattiasBuelens]

• Update to Android Gradle Plugin 9.1.0.
• Migrate from Groovy DSL to Kotlin DSL for all Gradle files
• Move shared build logic to a Gradle convention plugin. This is heavily inspired by the setup from Now in Android.
• Update to Dokka 2.2.0, following their migration guide.
• Rework the updateVersion task to write to the Gradle version catalog in gradle/libs.version.toml.