chore(deps): bump django in /appengine/standard_python3/django (GoogleCloudPlatform#13660)

Bumps [django](https://github.com/django/django) from 5.1.8 to 5.1.15.
- [Commits](django/django@5.1.8...5.1.15)

---
updated-dependencies:
- dependency-name: django
  dependency-version: 5.1.15
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

appengine/standard_python3/django/requirements.txt

@@ -1,4 +1,4 @@
-Django==5.1.8; python_version >= "3.10"
+Django==5.1.15; python_version >= "3.10"
 Django==4.2.17; python_version >= "3.8" and python_version < "3.10"
 Django==3.2.25; python_version < "3.8"
 django-environ==0.10.0