eve/downgrade: Deactivate downgrade in CI tests

Currently downgrade does not work because we downgrade all Kubernetes
objects before downgrading the API server of the bootstrap node.
Kubernetes 1.16 removed the support of some APIs that we use in the 2.4
branch, hence we cannot downgrade without re-ordering the whole process.

See: #2058

eve/main.yml

@@ -425,7 +425,8 @@ stages:
           name: Trigger single-node and multiple-nodes steps with built ISO
           stage_names:
             - single-node-upgrade-centos
-            - single-node-downgrade-centos
+            # Downgrade is deactivated on this branch due to #2058
+            #- single-node-downgrade-centos
             - single-node-install-rhel
             - multiple-nodes-centos
       - ShellCommand: *add_final_status_artifact_success