Hello Mel,
Welcome to Microsoft Q&A.
The first-generation Surface Go supports the Windows 10 system. I can notice that there is a version compatible with Windows 10 in the official driver and firmware download address:
Download Surface Go [Wi-Fi] Drivers and Firmware from Official Microsoft Download Center
However, it cannot be upgraded to Windows 11. You can notice here that this model of Surface is not on the list that supports Windows 11:
Which Surface devices can be upgraded to Windows 11? - Microsoft Support
As for why this device keeps receiving prompts to upgrade to Windows 11, I think it should be a mistake. I'm very sorry that I'm not from Microsoft, so I can't offer any explanation on behalf of Microsoft. However, I think it's necessary to provide feedback directly to Microsoft through the Feedback Hub app:
Send feedback to Microsoft with the Feedback Hub app - Microsoft Support
This is clearly a mistake. If you are willing to share the feedback link here, I will vote for you.
Best Regards,
Ian | Microsoft Q&A Support Specialist