So this drop-down is populated by a metadata file that is baked into Rancher at build time. Now Rancher will reach out to kontainer-driver-metadata/data at dev-v2.7 · rancher/kontainer-driver-metadata · GitHub every 6 hours looking for updated RKE1/2 and k3s versions. So if this is an air gapped environment IE Rancher can’t reach the internet, that will not get updated. You can of course download that json data (it’s like ~5MB) and host it internally and go to Rancher settings and change the metadata URL. I also built a image for doing this too. support-tools/rancher-metadata-syncer at master · rancherlabs/support-tools · GitHub.
Note: 1.24.8 is the latest certified version so anything newer will need to be made manually outside of Rancher then imported but of course there is no guarantee that you won’t have problems.
You can find the official support matrix at Support matrix | SUSE