chore(deps): update dependency django to v3.0.8 (#4219)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [Django](https://www.djangoproject.com/) ([source](https://togithub.com/django/django), [changelog](https://togithub.com/django/django/tree/master/docs/releases)) | patch | `==3.0.7` -> `==3.0.8` |
| [Django](https://www.djangoproject.com/) ([source](https://togithub.com/django/django), [changelog](https://togithub.com/django/django/tree/master/docs/releases)) | major | `==1.11.29` -> `==3.0.8` |

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### Release Notes

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

### [`v3.0.8`](https://togithub.com/django/django/compare/3.0.7...3.0.8)

[Compare Source](https://togithub.com/django/django/compare/3.0.7...3.0.8)

</details>

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appengine/standard/django/requirements.txt

@@ -1,2 +1,2 @@
 PyMySQL==0.9.3
-Django==1.11.29 # needs to stay under 2.0.0 for Python 2 support
+Django==1.11.29; python_version < '3.0'  # needs to stay under 2.0.0 for Python 2 support

appengine/standard_python3/django/requirements.txt

@@ -1,2 +1,2 @@
-Django==3.0.7
+Django==3.0.8
 PyMySQL==0.9.3

appengine/standard_python37/django/requirements.txt

@@ -1,2 +1,2 @@
-Django==3.0.7
+Django==3.0.8
 PyMySQL==0.9.3