Add a known issue for Elastic Agents getting stuck in an upgrade scheduled state (#2324)

This PR adds a known issue for Elastic Agents getting stuck in an
`upgrade scheduled` state.

Contributes to closing
#2285
Related to elastic/ingest-docs#1841 (PR for the
8.19 release)
Related to elastic/ingest-docs#1842 (PR for the
8.18.x releases)

Author: vishaangelova

release-notes/fleet-elastic-agent/known-issues.md

@@ -17,6 +17,38 @@ Known issues are significant defects or limitations that may impact your impleme
 
 % :::
 
+:::{dropdown} {{agents}} remain in an "Upgrade scheduled" state
+
+**Applies to: {{agent}} 8.18.0, 8.18.1, 8.18.2, 8.18.3, 8.18.4, 8.19.0, 9.0.0, 9.0.1, 9.0.2, 9.0.3, 9.1.0**
+
+On July 2, 2025, a known issue was discovered where {{agent}} remains in an `Upgrade scheduled` state when a scheduled {{agent}} upgrade is cancelled. Attempting to restart the upgrade on the UI returns an error: `The selected agent is not upgradeable: agent is already being upgraded.`.
+
+For more information, check [Issue #8778](https://github.com/elastic/elastic-agent/issues/8778).
+
+**Workaround**
+
+Call the [Upgrade an agent](https://www.elastic.co/docs/api/doc/kibana/operation/operation-post-fleet-agents-agentid-upgrade) endpoint of the Kibana Fleet API with the `force` parameter set to `true` to force-upgrade the {{agent}}:
+
+```powershell
+curl --request POST \
+  --url https://<KIBANA_HOST>/api/fleet/agents/<AGENT_ID>/upgrade \
+  --user "<SUPERUSER_NAME>:<SUPERUSER_PASSWORD>" \
+  --header 'Content-Type: application/json' \
+  --header 'kbn-xsrf: true' \
+  --data '{"version": "<VERSION>","force": true}'
+```
+
+To force-upgrade multiple {{agents}}, call the [Bulk upgrade agents](https://www.elastic.co/docs/api/doc/kibana/operation/operation-post-fleet-agents-bulk-upgrade) endpoint of the Kibana Fleet API with the `force` parameter set to `true`:
+
+```powershell
+curl --request POST \
+  --url https://<KIBANA_HOST>/api/fleet/agents/bulk_upgrade \
+  --user "<SUPERUSER_NAME>:<SUPERUSER_PASSWORD>" \
+  --header 'Content-Type: application/json' \
+  --header 'kbn-xsrf: true' \
+  --data '{"version": "<VERSION>","force": true,"agents":["<AGENT_IDS>"]}'
+```
+:::
 
 :::{dropdown} [Windows] {{agent}} is unable to re-enroll into {{fleet}}