Bump django from 3.1.8 to 3.1.14 in /examples/cookbook-plain (#1282)

Bumps [django](https://github.com/django/django) from 3.1.8 to 3.1.14.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.8...3.1.14)

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

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

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

examples/cookbook-plain/requirements.txt

@@ -1,4 +1,4 @@
 graphene>=2.1,<3
 graphene-django>=2.1,<3
 graphql-core>=2.1,<3
-django==3.1.8
+django==3.1.14