fix: backend/requirements/base.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-1316216

backend/requirements/base.txt

@@ -1,6 +1,6 @@
 pytz==2020.1  # https://github.com/stub42/pytz
 python-slugify==4.0.0  # https://github.com/un33k/python-slugify
-Pillow==7.1.2  # https://github.com/python-pillow/Pillow
+Pillow==8.3.0  # https://github.com/python-pillow/Pillow
 argon2-cffi==20.1.0  # https://github.com/hynek/argon2_cffi
 whitenoise==5.1.0  # https://github.com/evansd/whitenoise
 redis==3.5.0  # https://github.com/andymccurdy/redis-py