📌(backend) pin celery to version<5.6.0

Since celery version 5.6.0 we have trouble with retrying tasks and it is
impactig the malware_detection workflow. We have to use version 5.5.3
while we found the issue.

src/backend/pyproject.toml

@@ -28,7 +28,7 @@ dependencies = [
     "beautifulsoup4==4.14.3",
     "boto3==1.42.17",
     "Brotli==1.2.0",
-    "celery[redis]==5.6.0",
+    "celery[redis]==5.5.3",
     "django-configurations==2.5.1",
     "django-cors-headers==4.9.0",
     "django-countries==8.2.0",