docs: replace deprecated ujson with orjson in example

[thecaptain789]

Fixes #10795

The ujson library has been deprecated and put into maintenance-only mode. This PR updates the client_quickstart documentation to use orjson instead.

Changes:

• Replace ujson import with orjson
• Add wrapper function to decode bytes to string (orjson.dumps returns bytes)
• Update the note to describe orjson instead of ujson

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing thecaptain789:docs-replace-ujson-with-orjson (da979dd) with master (4bb9e6e)}

Summary

✅ 59 untouched benchmarks

[Dreamsorcerer]

I would expect the .decode() call to lose any performance gains here. So, let's look at #11989 first.