Document npm upstreaming (#24109)

* Swap ecosystem toggles screenshot

Swapped in new screenshot with NPM upstreaming

* Initial description of npm upstreaming

Co-authored-by: Ethan Palm <56270045+ethanpalm@users.noreply.github.com>

Author: galaxyeden

assets/images/enterprise/3.1/site-admin-settings/ecosystem-toggles.png

@@ -23,8 +23,9 @@ To prevent new packages from being uploaded, you can set an ecosystem you previo
 {% data reusables.enterprise_site_admin_settings.access-settings %}
 {% data reusables.enterprise_site_admin_settings.management-console %}
 {% data reusables.enterprise_site_admin_settings.packages-tab %}
-1. Under "Ecosystem Toggles", for each package type, select **Enabled**, **Read-Only**, or **Disabled**.
-  ![Ecosystem toggles](/assets/images/enterprise/site-admin-settings/ecosystem-toggles.png)
+1. Under "Ecosystem Toggles", for each package type, select **Enabled**, **Read-Only**, or **Disabled**.{% ifversion ghes > 3.1 %}
+  ![Ecosystem toggles](/assets/images/enterprise/site-admin-settings/ecosystem-toggles.png){% else %}
+  ![Ecosystem toggles](/assets/images/enterprise/3.1/site-admin-settings/ecosystem-toggles.png){% endif %}
 {% data reusables.enterprise_management_console.save-settings %}
 
 {% ifversion ghes = 3.0 or ghes > 3.0 %}
@@ -42,6 +43,8 @@ To allow network connections to the npm registry, you will need to configure net
 
 Note that connections to `registry.npmjs.com` traverse through the Cloudflare network, and subsequently do not connect to a single static IP address; instead, a connection is made to an IP address within the CIDR ranges listed here: https://www.cloudflare.com/ips/.
 
+If you wish to enable npm upstream sources, select `Enabled` for `npm upstreaming`.
+
 {% endif %}
 
 ## Next steps