build(deps): update httpx requirement from >=0.24 to >=0.28.1

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@0.24.0...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]

requirements.txt

@@ -1,5 +1,5 @@
 dfx-agentguard>=0.2.0
 fastapi>=0.100
 uvicorn>=0.20
-httpx>=0.24
+httpx>=0.28.1
 pyyaml>=6.0