Skip CG for version 6 branches (#7671)

Author: jander-msft

eng/pipelines/dotnet-monitor-cg.yml

@@ -8,6 +8,10 @@ schedules:
     - shipped/v7.3
     - shipped/v7.2
     - shipped/v7.1
+    - shipped/v6.3
+    - shipped/v6.2
+    - shipped/v6.1
+    - shipped/v6.0
   always: true
 
 trigger: none