chore(deps): update httpx requirement in /api-server

Updates the requirements on [httpx](https://github.com/encode/httpx) to permit the latest version.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.27.0...0.28.1)

---
updated-dependencies:
- dependency-name: httpx
  dependency-version: 0.28.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

api-server/requirements.txt

@@ -1,4 +1,4 @@
 fastapi>=0.115.0
 uvicorn[standard]>=0.32.0
-httpx>=0.27.0
+httpx>=0.28.1
 pydantic>=2.0.0