I would not recommend jumping directly from Rancher v2.3.x to v2.6.x as this is an unsupported upgrade path. Officially Rancher only supports jumping one minor version at a time IE v2.3.x to v2.5.x but as someone who is part of the Rancher Support team I would highly encourage you to slowly upgrade Rancher without jumping minor versions IE v2.3.x to v2.4.x then allow some time for burn-in. Then upgrade from v2.4.x to v2.5.x and repeat until you are at v2.6.3. The same also goes for upgrading k8s versions. Rancher and upstream only test and support jumping one minor version IE v1.17.x to v1.19.x.
This is all covered by my Master class which can be found at Kubernetes-Master-Class/rancher-k8s-upgrades at main · mattmattox/Kubernetes-Master-Class · GitHub