deps(deps): bump httpx from 0.26.0 to 0.28.1 in /backend

Bumps [httpx](https://github.com/encode/httpx) from 0.26.0 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@0.26.0...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] <support@github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -8,7 +8,7 @@ psycopg2-binary==2.9.9
 redis==5.0.1
 alembic==1.13.1
 python-dotenv==1.0.0
-httpx==0.26.0
+httpx==0.28.1
 stellar-sdk==9.0.0
 PyJWT==2.8.0
 pycryptodome==3.20.0