Custom Update Center settings not carried over

### Apache NetBeans version

Apache NetBeans 28

### What happened

When I upgraded from NB 26 to 28, a custom Update Center source that I had in 26 did not carry over to 28 even though I chose to import my settings.

### Language / Project Type / NetBeans Component

Upgrade

### How to reproduce

In NB 26, add a custom Update Center source. I used
JRebel for Netbeans
https://dl.zeroturnaround.com/jrebel/netbeans/updates.xml

Upgrade to NB 28, choosing to import settings. Note that the custom entry is missing.

### Did this work correctly in an earlier version?

No / Don't know

### Operating System

Xubuntu 24.04

### JDK

17.0.16; OpenJDK 64-Bit Server VM 17.0.16+8-Ubuntu-0ubuntu124.04.1

### Apache NetBeans packaging

Apache NetBeans binary zip

### Anything else

_No response_

### Are you willing to submit a pull request?

No

Start by reproducing the settings import from NetBeans 26 to 28 with the custom JRebel Update Center source described in the issue, then trace the upgrade handling for Update Center entries. Done means the custom source is retained after choosing to import settings.

Written by the indexing model from the issue text.