Add mcg on ocp-4.21 and drop ocp-4.18

Author: mbaldessari

ci-triggers/multicloud-gitops.yaml

@@ -8,22 +8,22 @@ triggers:
   branches:
   - name: main
     versions:
+    - '4.21'
     - '4.20'
     - '4.19'
-    - '4.18'
 
   openshift:
-  - version: '4.20'
+  - version: '4.21'
     branch: main
     platforms:
       - aws
     buildType: stable
-  - version: '4.19'
+  - version: '4.20'
     branch: main
     platforms:
       - aws
     buildType: stable
-  - version: '4.18'
+  - version: '4.19'
     branch: main
     platforms:
       - azure