[Guardrails] Migration of StorageBackend from CouchStore to Magma wasn't allowed when the bucket was hitting CouchStore max data per bucket guardrail

Steps:

1. Initialise a 2 node cluster running just the KV service.
2. Configure couchstoreMaximum=0.002 in http://172.23.106.202:8091/settings/resourceManagement.
3. Create a CouchStore bucket named 'default' with 512MB RAM per node.
4. Load until the bucket would hit the max data per bucket guardrail. The data size of the bucket after hitting the guardrail was over 2GB on both the nodes.
5. Tried migration by updating the storageBackend to Magma.
6. Got the following error:

{"errors":{"storageBackend":"Storage mode migration is not allowed when data size per node is above the configured maximum: 0.002TB"

# But since, magmaMaximum is set to 16TB and after migration to Magma, bucket wouldn't be hitting the guardrail, hence migration should be allowed.

This is a similar issue to MB-59042