refactor: clean up deprecated APIs for v6.1.0

[hyochan]

Changes

• Renamed getAvailablePurchases() to getActivePurchases() for clarity
• Renamed getPurchaseHistory() to getPurchaseHistories() for consistency
• Removed all deprecated methods marked for removal in v6.0.0
• Cleaned up legacy code and unused methods
• Updated example code to use new APIs

Breaking Changes

• getAvailablePurchases() → getActivePurchases()
• Removed deprecated methods: initialize(), checkSubscribed(), getAvailablePurchasesAsync(), etc.

Migration Guide

• Replace getAvailablePurchases() with getActivePurchases()
• Replace getPurchaseHistory() with getPurchaseHistories()
• Use initConnection() instead of initialize()
• Use finishTransaction() instead of deprecated platform-specific methods