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:development
...

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

Author: dependabot[bot]

dev-requirements.txt

@@ -8,7 +8,7 @@ wheel
 # used in unit test only
 opentelemetry-sdk
 opentelemetry-instrumentation-grpc
-httpx>=0.24
+httpx>=0.28.1
 pyOpenSSL>=26.0.0
 # needed for type checking
 Flask>=1.1