Bump httpx from 0.27.0 to 0.28.1

Bumps [httpx](https://github.com/encode/httpx) from 0.27.0 to 0.28.1.
- [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
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

app/backend/requirements.txt

@@ -124,7 +124,7 @@ hpack==4.1.0
     # via h2
 httpcore==1.0.9
     # via httpx
-httpx==0.27.0
+httpx==0.28.1
     # via
     #   microsoft-kiota-http
     #   msgraph-core