ci/scalability: remove test on Rancher Manager Devel

It's not very useful to keep this test, as scalability is not our main
focus and moreover the Devel version of RM is already tested in regular
CI. Better to focus on RM Stable version for this test.

Signed-off-by: Loic Devulder <ldevulder@suse.com>

.github/workflows/cli-k3s-scalability-matrix.yaml

@@ -32,7 +32,7 @@ jobs:
       matrix:
         k8s_downstream_version: ${{ fromJSON(format('[{0}]', inputs.k8s_downstream_version || '"v1.27.13+k3s1"')) }}
         k8s_upstream_version: ${{ fromJSON(format('[{0}]', inputs.k8s_upstream_version || '"v1.27.13+k3s1"')) }}
-        rancher_version: ${{ fromJSON(format('[{0}]', inputs.rancher_version || '"stable/latest","latest/devel/2.8"')) }}
+        rancher_version: ${{ fromJSON(format('[{0}]', inputs.rancher_version || '"stable/latest"')) }}
     uses: ./.github/workflows/master_e2e.yaml
     secrets:
       credentials: ${{ secrets.GCP_CREDENTIALS }}