Bump django from 3.0.7 to 3.1.6 in /examples/cookbook-plain (#1149)

Bumps [django](https://github.com/django/django) from 3.0.7 to 3.1.6. - [Release notes](https://github.com/django/django/releases) - [Commits](django/django@3.0.7...3.1.6) Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

examples/cookbook-plain/requirements.txt

@@ -1,4 +1,4 @@
 graphene>=2.1,<3
 graphene-django>=2.1,<3
 graphql-core>=2.1,<3
-django==3.0.7
+django==3.1.6