Bump djangorestframework from 3.9.3 to 3.11.2 in /services/webapp

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.3 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/3.9.3...3.11.2

)

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