Add release note for known issue with Manage GHES API (#38319)

Author: mattpollard

data/release-notes/enterprise-server/3-9/0-rc1.yml

@@ -383,6 +383,8 @@ sections:
   known_issues:
     - |
       If you upgrade from {% data variables.product.prodname_ghe_server %} 3.7 or 3.8 to 3.9, the database server on your instance will be upgraded from MySQL 5.7 to MySQL 8.0. I/O utilization will increase as a result, and in some cases this may affect your instance's performance. Do not upgrade to this RC in a production environment, and ensure that you take and verify a backup of the instance before upgrading to the GA release. For more information, see "[AUTOTITLE](/admin/enterprise-management/updating-the-virtual-machine-and-physical-resources/known-issues-with-upgrades-to-your-instance)."
+    - |
+      {% data reusables.release-notes.manage-api-unreachable %}
     - |
       On an instance in a cluster configuration, after you upgrade nodes other than the primary MySQL node and before you upgrade the primary MySQL node, the following output may appear multiple times after you run `ghe-config-apply`.
 

data/reusables/release-notes/manage-api-unreachable.md

@@ -0,0 +1,9 @@
+After upgrading an existing instance to GitHub Enterprise Server 3.9, the Manage GitHub Enterprise Server API is unavailable. To enable the API, SSH into the instance and run the following commands.
+
+```shell copy
+sudo mkdir -p /data/ghes-manage-gateway/current
+sudo chown -R ghes-manage-gateway:ghes-manage-gateway /data/ghes-manage-gateway/current
+sudo systemctl restart ghes-manage-gateway ghes-manage-gateway-consul
+```
+
+For more information about the Manage GitHub Enterprise Server API, see "[Manage GitHub Enterprise Server](/rest/enterprise-admin/manage-ghes?apiVersion=2022-11-28)" in the REST API documentation. [Updated: 2023-06-22]