feat: upgrade BillingClient to 8.0.0

[ghsdh3409]

🔧 Changes

BillingClient 8.0.0 Migration

• Upgrade: billing-ktx:7.1.1 → 8.0.0
• Fix: Replace deprecated enablePendingPurchases() with parameterized version
• Fix: Replace removed queryPurchaseHistoryAsync() with queryPurchasesAsync()
• Upgrade: Kotlin 1.8.20 → 2.1.0 (required for billing 8.0.0 compatibility)

🐛 Problem Solved

Fixes NoSuchMethodError when customer apps use BillingClient 8.0.0:
java.lang.NoSuchMethodError: No virtual method enablePendingPurchases()

✅ Compatibility

Requirements

• Minimum: BillingClient 8.0.0+ now required
• Kotlin: 2.1.0+ (upgraded from 1.8.20)

Customer Impact

• Existing customers (billing 7.x): ✅ Auto-upgraded to 8.0.0 via Gradle dependency resolution
• New customers (billing 8.0.0+): ✅ NoSuchMethodError crash resolved
• Functional behavior: ✅ Maintained (one-time + subscription tracking continues)