chore(deps): update dependency django to v3.1.6 (GoogleCloudPlatform#5316)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [Django](https://www.djangoproject.com/) ([source](https://togithub.com/django/django), [changelog](https://togithub.com/django/django/tree/master/docs/releases)) | `==3.1.5` -> `==3.1.6` | [![age](https://badges.renovateapi.com/packages/pypi/Django/3.1.6/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/Django/3.1.6/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/Django/3.1.6/compatibility-slim/3.1.5)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/Django/3.1.6/confidence-slim/3.1.5)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>django/django</summary>

### [`v3.1.6`](https://togithub.com/django/django/compare/3.1.5...3.1.6)

[Compare Source](https://togithub.com/django/django/compare/3.1.5...3.1.6)

</details>

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appengine/standard_python3/django/requirements.txt

@@ -1,2 +1,2 @@
-Django==3.1.5
+Django==3.1.6
 PyMySQL==1.0.2

run/django/requirements.txt

@@ -1,4 +1,4 @@
-Django==3.1.5
+Django==3.1.6
 django-storages[google]==1.11.1
 django-environ==0.4.5
 psycopg2-binary==2.8.6