Can 8th Gen iPad be upgraded to support 5G?

It is impossible to increase change or upgrade the internal hardware of your iPad - as this is fixed during manufacture; what you have is fixed for the life of your device.  For Cellular data connectivity, whether you have a WiFi Only or WiFi+Cellular model of iPad, you can use the cellular hotspot feature of a suitable 5G mobile phone or 5G Cellular Router (sometimes known as a MiFi router).

If you have a WiFi only model, it can not be modified to provide cellular connectivity.