📌(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.

renovate.json

@@ -31,6 +31,13 @@
       "matchPackageNames": ["django"],
       "allowedVersions": "<6.0.0"
     },
+    {
+
+      "groupName": "allowed celery versions",
+      "matchManagers": ["pep621"],
+      "matchPackageNames": ["celery"],
+      "allowedVersions": "<5.6.0"
+    },
     {
       "enabled": false,
       "groupName": "ignored js dependencies",