Bump azure-identity from 1.13.0 to 1.16.1 in /src/prepdocs

Bumps [azure-identity](https://github.com/Azure/azure-sdk-for-python) from 1.13.0 to 1.16.1.
- [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_1.13.0...azure-identity_1.16.1)

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

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

src/prepdocs/requirements.txt

@@ -1,5 +1,5 @@
 pypdf==3.17.0
-azure-identity==1.13.0
+azure-identity==1.16.1
 azure-search-documents==11.4.0b6
 azure-ai-formrecognizer==3.2.1
 azure-storage-blob==12.14.1