Upgrade Xamarin.Android.Google.BillingClient to 8.3.0.2

[Copilot]

Upgrades the Google Play Billing Library binding from 8.1.0 to 8.3.0.2 and applies all breaking-change mitigations required by the PBL 8.1–8.3 release notes.

Dependency changes

• Xamarin.Android.Google.BillingClient: 8.1.0 → 8.3.0.2
• Xamarin.AndroidX.Lifecycle.LiveData.Core: 2.9.3 → 2.10.0.2 (transitive conflict introduced by BillingClient 8.3.0.2)

API migration — subscription replacement mode (PBL 8.1)

SubscriptionUpdateParams.SetSubscriptionReplacementMode() is deprecated in PBL 8.1. Replacement mode now belongs on ProductDetailsParams via SubscriptionProductReplacementParams:

// Before (deprecated)
var updateParams = BillingFlowParams.SubscriptionUpdateParams.NewBuilder()
    .SetOldPurchaseToken(token)
    .SetSubscriptionReplacementMode((int)prorationMode)
    .Build();

// After
var replacementParams = SubscriptionProductReplacementParams.NewBuilder()
    .SetReplacementMode((int)prorationMode)
    .Build();

var prodDetailsParams = BillingFlowParams.ProductDetailsParams.NewBuilder()
    .SetProductDetails(skuDetails)
    .SetOfferToken(offerToken)
    .SetSubscriptionProductReplacementParams(replacementParams)
    .Build();

var updateParams = BillingFlowParams.SubscriptionUpdateParams.NewBuilder()
    .SetOldPurchaseToken(token)
    .Build(); // no replacement mode here anymore

Other required changes (PBL 8.1)

• Android minSdkVersion: 21 → 23 (SupportedOSPlatformVersion in .csproj)
• New enum value: SubscriptionProrationMode.KeepExisting = 6 — keeps the original payment schedule when replacing a subscription