Bump django from 2.2.13 to 2.2.21 in /demo

Bumps [django](https://github.com/django/django) from 2.2.13 to 2.2.21.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.13...2.2.21)

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

Signed-off-by: dependabot[bot] <[email protected]>

demo/requirements.txt

@@ -1,2 +1,2 @@
-Django==2.2.13
+Django==2.2.21
 psycopg2-binary==2.8.2