build(deps): Update idna requirement from >=3.15 to >=3.16

Updates the requirements on [idna](https://github.com/kjd/idna) to permit the latest version.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.md)
- [Commits](kjd/idna@v3.15...v3.16)

---
updated-dependencies:
- dependency-name: idna
  dependency-version: '3.16'
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -15,7 +15,7 @@ google-cloud-datastore
 google-cloud-pubsub
 gunicorn>=26.0.0
 httpx>=0.28.1
-idna>=3.15
+idna>=3.16
 ipaddress
 itsdangerous
 jinja2>=3.1.6