architecture: move from `.gradle` to `.gradle.kts` (Kotlin DSL) for build scripts/logic

[david-allison]

Mostly to keep up with Android ecosystem changes.

Type safety is a benefit, as well as our contributors being more familiar with Kotlin than Groovy.

• AnkiDroid/build.gradle
• AnkiDroid/jacoco.gradle: refactor(build): convert jacoco.gradle Kotlin DSL #20911
• AnkiDroid/robolectricDownloader.gradle refactor(build): convert robolectricDownloader to Kotlin DSL #20937
• lint.gradle
• jacocoSupport.gradle

• blocked on build: Introduce Gradle Convention Plugins #20778