Better Depth Tracking & Date Parsing

• Reverse course and replace FHIRDecodingDepthTracker with JSONDecoder.maxFHIRModelsDepth. There now is on-by-default depth tracking, see FHIRDecodingDepth.swift, implemented on top of Decoder. Thanks to Lukas Kollmer for the analysis and proposal in #45
• Lower the deployment target back to iOS 15 / macOS 12 / watchOS 8 / tvOS 15, since dropping the custom depth tracker removed the OSAllocatedUnfairLock dependency that had raised it in 0.9.2 (resolves #44)
• Implement a custom, specialized, performance & safety -optimized date/time parser