[gtk4]Switching on "use as default" for a printer does not toggle off the switch for other printers

### What Happened?

With two printer drivers installed the first printer was set as default. The second printer was then switched to be default but checking back on the first printer, the default switch was still active.

Moreover exiting and re-entering Switchboard showed that the default printer was not changed.

However if the first printer was manually switched off as default and the second printer switched on then this setting did persist.

### Steps to Reproduce

1. Install two printers
2. Set the first as default and the second not default
3. Set the second as default.
4. Return to the "Settings & Supplies" page of the first printer - it is still default
5. Return to the "Settings & Supplies" page of the second printer - it is also default
6. Exit Switchboard
7. Restart Switchboard and enter the Printers plug
8. The first printer is still default
9. Switch the first printer "Use as default" off
10. Switch the second printer "Use as default" on
11. Exit Switchboard
12. Restart Switchboard and enter the Printers plug
13. The second printer is still default.

### Expected Behavior

Switching the "Use as default" switch on for a printer should switch off the same switch for all other printers. It might be more discoverable to be able to set the default printer from the sidebar.

### OS Version

7.x (Early Access)

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

_No response_