fix: remove legacy StoreKit support claims and update minimum platform versions

- Remove misleading 'Legacy StoreKit support (iOS 13-14)' from README
- Update Package.swift platform requirements to match actual implementation:
  - iOS 15.0+ (was 13.0+)
  - macOS 14.0+ (was 10.15+)
  - tvOS 15.0+ (was 13.0+)
  - watchOS 8.0+ (was 6.0+)
- Update README requirements table to reflect accurate minimum versions
- Current implementation only supports StoreKit 2 with @available(iOS 15.0+)

Author: hyochan

Package.swift

@@ -5,10 +5,10 @@ import PackageDescription
 let package = Package(
     name: "OpenIAP",
     platforms: [
-        .iOS(.v13),
-        .macOS(.v10_15),
-        .tvOS(.v13),
-        .watchOS(.v6)
+        .iOS(.v15),
+        .macOS(.v14),
+        .tvOS(.v15),
+        .watchOS(.v8)
     ],
     products: [
         .library(

README.md

@@ -21,7 +21,6 @@ Visit [**openiap.dev**](https://openiap.dev) for complete documentation, guides,
 ## ✨ Features
 
 - ✅ **StoreKit 2** support with full iOS 15+ compatibility
-- ✅ **Legacy StoreKit** fallback for iOS 13-14
 - ✅ **Cross-platform** support (iOS, macOS, tvOS, watchOS)
 - ✅ **Unified API** following OpenIAP specification
 - ✅ **Product management** with intelligent caching
@@ -35,10 +34,10 @@ Visit [**openiap.dev**](https://openiap.dev) for complete documentation, guides,
 
 | Platform | Minimum Version |
 |----------|-----------------|
-| iOS | 13.0+ |
-| macOS | 10.15+ |
-| tvOS | 13.0+ |
-| watchOS | 6.0+ |
+| iOS | 15.0+ |
+| macOS | 14.0+ |
+| tvOS | 15.0+ |
+| watchOS | 8.0+ |
 | Swift | 5.9+ |
 
 ## 📦 Installation