Migrate LinuxScanner from Newtonsoft.Json to System.Text.Json (#1567)

- `LinuxScanner`:
  - Replace `JsonConvert.SerializeObject` with `JsonSerializer.Serialize`
  - Replace `JsonConvert.DeserializeObject` with `SyftOutput.FromJson()`
  - Remove `Newtonsoft.Json` using directive
- `SyftOutput`:
  - Regenerate from JSON schema using quicktype.io with `System.Text.Json`
  - Add `[JsonPropertyName]` attributes for proper property mapping
  - Add `[JsonIgnore]` attributes for null value handling
  - Add `FromJson()` helper method with case-insensitive parsing
  - Enable nullable reference types with appropriate pragmas
- `LinuxScannerTests`:
  - Convert test JSON strings from escaped C# strings to raw literals
  - Fix property name casing to match schema (`versionID` vs `versionId`)
  - Remove trailing commas that were previously tolerated

See #231

Author: JamieMagee