Bump h2 from 4.1.0 to 4.3.0 in /app/backend

Bumps [h2](https://github.com/python-hyper/h2) from 4.1.0 to 4.3.0.
- [Changelog](https://github.com/python-hyper/h2/blob/master/CHANGELOG.rst)
- [Commits](python-hyper/h2@v4.1.0...v4.3.0)

---
updated-dependencies:
- dependency-name: h2
  dependency-version: 4.3.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

app/backend/requirements.txt

@@ -17,7 +17,7 @@ colorama==0.4.6
 cryptography==44.0.1
 Flask==3.0.1
 h11==0.14.0
-h2==4.1.0
+h2==4.3.0
 hpack==4.0.0
 Hypercorn==0.16.0
 hyperframe==6.0.1