fix: apply Kotlin Gradle Plugin only when AGP has no built-in Kotlin

[utkrishtsahu]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

Android Gradle Plugin 9 compiles Kotlin itself ("built-in Kotlin") and fails the build when a module also applies the Kotlin Gradle Plugin (KGP):

Failed to apply plugin 'org.jetbrains.kotlin.android'
The 'org.jetbrains.kotlin.android' plugin is no longer required for Kotlin support since AGP 9.0.

Flutter 3.47 scaffolds apps on AGP 9.1.0, so any app depending on auth0_flutter currently has to opt out with android.builtInKotlin=false — which in turn forces every other plugin in the app back onto KGP.

This changes auth0_flutter/android/build.gradle to apply KGP only where AGP will not compile Kotlin itself:

• AGP ≤ 8 (example app / CI): unchanged — KGP is applied exactly as before.
• AGP 9+, built-in Kotlin (the default): KGP is not applied; AGP compiles the Kotlin sources.
• AGP 9+, host opted out with android.builtInKotlin=false: KGP is applied, as before.

The kotlinOptions { jvmTarget = '17' } block is KGP-only DSL and does not exist under built-in Kotlin, so it moves behind plugins.withId("org.jetbrains.kotlin.android") using JvmTarget.JVM_17; with built-in Kotlin, AGP derives the JVM target from compileOptions.targetCompatibility (already VERSION_17).

pluginManager.apply('kotlin-android') is used instead of apply plugin: on purpose: the Flutter tool detects KGP usage by regex-scanning plugin build scripts and would otherwise keep listing auth0_flutter in its KGP notice even on AGP 9, where this branch never runs. This mirrors the conditional apply shipped by firebase_core, stripe_android, and mobile_scanner.

No public API change.

Fixes #923.

🎯 Testing

Build-script only; no runtime code changes.

• AGP 8 (existing users / CI / example app) — verified end-to-end. Built the auth0_flutter/example app to a debug APK on AGP 8.10.1: ✓ Built build/app/outputs/flutter-apk/app-debug.apk. On AGP ≤ 8 the script takes the original agpMajor < 9 branch and applies KGP identically to before, so existing consumers are unaffected.

[coderabbitai]

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