Validate minimum deployment targets

Current deployment targets in Package.swift:
```
        .macOS(.v10_15),
        .iOS(.v13),
        .watchOS(.v7),
        .tvOS(.v14)
```

We should validate what the minimum targets that can work are. What OS versions have the correct SwiftUI Support?
We then need to ensure we have these also set in the xcodeproj for carthage, and the minimum version settings for CocoaPods.