Update pydantic-settings requirement in /code/backend

Updates the requirements on [pydantic-settings](https://github.com/pydantic/pydantic-settings) to permit the latest version.
- [Release notes](https://github.com/pydantic/pydantic-settings/releases)
- [Commits](pydantic/pydantic-settings@v2.5.2...v2.6.0)

---
updated-dependencies:
- dependency-name: pydantic-settings
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

code/backend/requirements.txt

@@ -8,4 +8,4 @@ openai~=1.51.0
 opentelemetry-instrumentation-aiohttp-client~=0.48b0
 opentelemetry-instrumentation-aiohttp-server~=0.48b0
 pydantic~=2.9.2
-pydantic-settings~=2.5.2
+pydantic-settings~=2.6.0