build: bump azure-identity from 1.20.0 to 1.21.0

Bumps [azure-identity](https://github.com/Azure/azure-sdk-for-python) from 1.20.0 to 1.21.0.
- [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_1.20.0...azure-identity_1.21.0)

---
updated-dependencies:
- dependency-name: azure-identity
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

src/requirements.txt

@@ -1,4 +1,4 @@
-azure-identity==1.20.0
+azure-identity==1.21.0
 # Flask[async]==2.3.2
 openai==1.65.2
 azure-search-documents==11.6.0b9