📌(backend) pin brevo-python to v2.x.x. in renovate

The SDK introduced breaking changes in newer versions. Lock the
dependency to v2.x.x to maintain compatibility.

renovate.json

@@ -36,6 +36,12 @@
         "matchPackageNames": ["django"],
         "allowedVersions": "<6.0.0"
     },
+    {
+        "groupName": "allowed brevo versions",
+        "matchManagers": ["pep621"],
+        "matchPackageNames": ["brevo-python"],
+        "allowedVersions": "<3.0.0"
+    },
     {
       "enabled": false,
       "groupName": "ignored js dependencies",