[Fleet] Fix warning icon not displaying on 8.7 (elastic#154119)

Originally reported in
elastic#152234 (comment)

With EUI v76, `warning` icon type was introduced:
elastic#152506

However, the EUI upgrade was only applied on 8.8. The callout here that
uses the warning icon was a bug fix backported to 8.7 after the EUI
upgrade. In 8.7 the EUI version does not have the `warning` icon type,
causing a broken image to be rendered in this callout.

This PR reverts the icon to the legacy `alert` type will only be merged
into 8.7.

x-pack/plugins/fleet/public/applications/fleet/sections/settings/components/edit_output_flyout/confirm_update.tsx

@@ -67,7 +67,7 @@ const ConfirmDescription: React.FunctionComponent<ConfirmDescriptionProps> = ({
       <>
         <EuiSpacer size="s" />
         <EuiCallOut
-          iconType="warning"
+          iconType="alert"
           color="warning"
           size="m"
           title={