Update azure-identity requirement in /code/backend

Updates the requirements on [azure-identity](https://github.com/Azure/azure-sdk-for-python) to permit the latest version.
- [Release notes](https://github.com/Azure/azure-sdk-for-python/releases)
- [Changelog](https://github.com/Azure/azure-sdk-for-python/blob/main/doc/esrp_release.md)
- [Commits](Azure/azure-sdk-for-python@azure-identity_1.17.1...azure-identity_1.19.0)

---
updated-dependencies:
- dependency-name: azure-identity
  dependency-type: direct:production
...

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

code/backend/requirements.txt

@@ -1,4 +1,4 @@
-azure-identity~=1.17.1
+azure-identity~=1.19.0
 azure-monitor-opentelemetry~=1.6.2
 aiohttp~=3.10.5
 botbuilder-core~=4.16.2