Bump django from 4.2 to 4.2.7 in /server/requirements (#856)

Bumps [django](https://github.com/django/django) from 4.2 to 4.2.7.
- [Commits](django/django@4.2...4.2.7)

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

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server/requirements/base.txt

@@ -2,7 +2,7 @@
 opendp==0.6.1
 
 # Web application
-Django==4.2
+Django==4.2.7
 asgiref>=3.3.2
 bleach==3.3.0
 certifi==2020.6.20