Bump djangorestframework from 3.11.2 to 3.15.2

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.11.2 to 3.15.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.11.2...3.15.2)

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

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

requirements.txt

@@ -1,6 +1,6 @@
 # Core
 Django>=2.2,<2.3
-djangorestframework==3.11.2
+djangorestframework==3.15.2
 
 # Tahoe requirements
 # TODO: Replace with tahoe-sites and edx-organizations==5.2.0 (https://github.com/appsembler/tahoe-figures-plugins/issues/3)