chore(deps): update httpx requirement

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]

reference-apps/shared/test-suite/requirements.txt

@@ -6,7 +6,7 @@ pytest-asyncio>=0.21.0
 pytest-timeout>=2.1.0
 
 # HTTP client for testing
-httpx>=0.24.0
+httpx>=0.28.1
 
 # Assertion helpers
 deepdiff>=6.3.0