fix: backend_app/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-6808823
- https://snyk.io/vuln/SNYK-PYTHON-CRYPTOGRAPHY-6592767

backend_app/requirements.txt

@@ -1,4 +1,4 @@
-aiohttp==3.8.1
+aiohttp==3.9.4
 amqp==2.5.2
 asgiref==3.3.4
 async-timeout==4.0.1
@@ -74,3 +74,4 @@ botocore==1.19.25
 django-cachalot==2.5.1
 python-memcached==1.59
 django-auth-adfs==1.9.5
+cryptography>=42.0.6 # not directly required, pinned by Snyk to avoid a vulnerability