Edit Subscriber when 2 subscribers belong to the same NS fails

[patriciareinoso]

Describe the bug

There is an unexpected behavior when we have 2 Subscribers that belong to the same NS and decide to change one of the to other NS.
We end up with 2 subscriber with the same imsi.

To Reproduce

1. Create 2 NS (NS1 and NS2)
2. Create Subscriber A in NS1
3. Create Subscriber B in NS1
4. Edit Subscriber A and change it to NS2
   -> Subscriber B disappears, Subscriber A is duplicated

Expected behavior

Subscriber A is changed to NS2
Subscriber B stays in NS1
Subscriber A is not duplicated

Screenshots

Logs

Environment

• Charm / library version (if relevant): edge
• Juju version (output from juju --version): 3.4
• Cloud Environment: microk8s
• Kubernetes version (output from kubectl version --short): 1.27

Additional context