chore(deps): update django-pgtrigger requirement in /api

Updates the requirements on [django-pgtrigger](https://github.com/Opus10/django-pgtrigger) to permit the latest version.
- [Changelog](https://github.com/Opus10/django-pgtrigger/blob/master/CHANGELOG.md)
- [Commits](https://github.com/Opus10/django-pgtrigger/compare/4.7.0...4.9.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

api/requirements.txt

@@ -7,7 +7,7 @@ django-cors-headers~=4.3.0
 djangorestframework~=3.14.0
 django-celery-email~=3.0.0
 django-netfields~=1.3.0
-django-pgtrigger~=4.7.0
+django-pgtrigger~=4.9.0
 django-prometheus~=2.3.1
 dnspython~=2.4.2
 httpretty~=1.0.5  # 1.1 breaks tests. Does not run in production, so stick to it.