Add note about deprecation of System.Json

[gewarren]

Fixes #3361.

Preview.

[opbld32]

Docs Build status updates of commit 269c703:

✅ Validation status: passed

File	Status	Preview URL	Details
includes/system-json.md	✅Succeeded	View
xml/System.Json/JsonArray.xml	✅Succeeded	View
xml/System.Json/JsonObject.xml	✅Succeeded	View
xml/System.Json/JsonPrimitive.xml	✅Succeeded	View
xml/System.Json/JsonType.xml	✅Succeeded	View
xml/System.Json/JsonValue.xml	✅Succeeded	View
xml/ns-System.Json.xml	✅Succeeded	View

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[BillWagner]

This LGTM @gewarren

I had one small suggestion, and then

[opbld33]

Docs Build status updates of commit 624fd86:

✅ Validation status: passed

File	Status	Preview URL	Details
includes/system-json.md	✅Succeeded	View
xml/System.Json/JsonArray.xml	✅Succeeded	View
xml/System.Json/JsonObject.xml	✅Succeeded	View
xml/System.Json/JsonPrimitive.xml	✅Succeeded	View
xml/System.Json/JsonType.xml	✅Succeeded	View
xml/System.Json/JsonValue.xml	✅Succeeded	View
xml/ns-System.Json.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel