Display error banner instead of warning (elastic#195913)

Closes [110155](elastic#110155)

## Summary
When the remote cluster creation has an error the banner displayed was a
warning instead of an error.

### How to reproduce?
1.) Navigate to Remote Clusters
2.) Create a new Remote Cluster
3.) Add information for a cluster except set the max number of
connections to 999999999999999999
4.) Try to save.

### Testing
No testing since this doesn't change any functionality.

<img width="1243" alt="110155"
src="https://github.com/user-attachments/assets/802aa9f3-f70a-44da-a135-8187ada2e78a">

x-pack/plugins/remote_clusters/public/application/sections/components/remote_cluster_form/remote_cluster_form.tsx

@@ -423,7 +423,7 @@ export class RemoteClusterForm extends Component<Props, State> {
 
       return (
         <Fragment>
-          <EuiCallOut title={message} iconType="cross" color="warning">
+          <EuiCallOut title={message} color="danger" iconType="error">
             {errorBody}
           </EuiCallOut>