AGP 9.0 deprecated the old applicationVariants API, so migration to the new Variant API is required.

Old Solution (Deprecated in AGP 9.0)

Previously, you could customize the output APK name via applicationVariants:

android {
    applicationVariants.all {
        outputs.all {
            val currentDate = SimpleDateFormat("yyyy.MM.dd").format(Date())
            val outputFileName =
                "${applicationId}_${buildType.name}_v${versionName}_${currentDate}.apk"
            (this as com.android.build.gradle.internal.api.BaseVariantOutputImpl).outputFileName =
                outputFileName
        }
    }
}

New Solution (androidComponents + onVariants)

The official recommendation is to use the new androidComponents API:

androidComponents {
    onVariants { variant ->
        variant.outputs.forEach { output ->
            val currentDate = SimpleDateFormat("yyyy.MM.dd").format(Date())
            val newName =
                "${variant.applicationId.get()}-${variant.name}-${output.versionName.get()}-${currentDate}.apk"
            output.outputFileName.set(newName)
        }
    }
}

Notes

The above example uses Kotlin DSL (build.gradle.kts).
applicationVariants is deprecated in AGP 9.0; it is recommended to migrate as soon as possible to avoid future upgrade errors.
When using SimpleDateFormat, you need to import java.text.SimpleDateFormat and java.util.Date at the top of the script.

AGP 9.0 Modify APK Packaging Name