Add a short -n for kubectl's --namespace

fixes #24078

--namespace is a very common flag for nearly every kubectl command we have.
We should claim -n for it.

docs/devel/kubectl-conventions.md

@@ -142,6 +142,7 @@ list when adding new short flags
 
   * `-f`: Resource file
     * also used for `--follow` in `logs`, but should be deprecated in favor of `-F`
+  * `-n`: Namespace scope
   * `-l`: Label selector
     * also used for `--labels` in `expose`, but should be deprecated
   * `-L`: Label columns