Simplify Utf8Json sample

Docs sample: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/use-utf8jsonreader#read-from-a-stream-using-utf8jsonreader

Proposed replacement: https://gist.github.com/richlander/d07cfa95509cb949a6fa3e89b218308a

---
#### Document Details

⚠ *Do not edit this section. It is required for learn.microsoft.com ➟ GitHub issue linking.*

* ID: ea0c3884-2451-338b-2ca0-93234597b623
* Version Independent ID: 8a70b3f5-1563-df48-6b98-452482ccd0b3
* Content: [How to use Utf8JsonReader in System.Text.Json](https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/use-utf8jsonreader#read-from-a-stream-using-utf8jsonreader)
* Content Source: [docs/standard/serialization/system-text-json/use-utf8jsonreader.md](https://github.com/dotnet/docs/blob/main/docs/standard/serialization/system-text-json/use-utf8jsonreader.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @gewarren
* Microsoft Alias: **gewarren**

---
[Associated WorkItem - 189692](https://dev.azure.com/msft-skilling/Content/_workitems/edit/189692)