The Community bit is important as I’ll note below and whilst this section of the forum is titled “SUSE Rancher Prime” there isn’t a non-Prime Community section so yes I’d say you’re in the right place. The alternative would be the Rancher Users Slack channel but I find that’s more for very specific questions and issues.
Since you are using the Community edition that limits the Rancher versions that you can use.
You have Rancher 2.8.3 and want to manage K3s 1.3.1 but to do that you need Rancher 2.10.x - see the Rancher Support Matrix at https://www.suse.com/suse-rancher/support-matrix/all-supported-versions/rancher-v2-10-1/
So how to get there? First you need to upgrade Rancher to 2.8.5 as the last Community release of Rancher 2.8.x (2.8.6 was first Prime-only release of 2.8.x). Then to 2.9.3 as last Community release of 2.9.x (2.9.4 was first Prime-only release of 2.9.x). Then to 2.10.3 as, you guessed it, last Community release of 2.10.x (2.10.4 was first Prime-only release of 2.10.x).
With that done you can upgrade your K3s cluster from 1.28.15 to 1.31 but via 1.29 and then 1.30 as it’s not supported to jump major versions.
Ditto with your Rancher hosting K3s although it supported to run Rancher 2.10.x on K3s 1.28.x but that will change with Rancher 2.11.x.
How did you originally install your K3s clusters as you should use the same way(s) to upgrade them?