Updating to JSONDecoder from JSONSerialization

[BritneyS]

What you did ❓

• Updated parse method: uses JSONDecoder instead of JSONSerialization

How you did it ✅

• Made model classes (LaunchGateConfiguration, UpdateConfiguration, and AlertConfiguration) conform to Decodable
• Manually decoded the LaunchGateConfiguration and UpdateConfiguration classes with decoder initializers
• Rewrote parse method to use JSONDecoder in order to parse the root object (LaunchGateConfiguration type) and its nested objects (UpdateConfiguration and AlertConfiguration)
• Removed error enum, since Decodable classes will throw the appropriate errors

How to test it 🔬

• Build and run the Example app
• An alert should pop up that reads "An update is required to continue using this app."

Screenshots (if applicable) 📷