[DOCS] Update changelog

docs/CHANGELOG.asciidoc

@@ -27,6 +27,59 @@ For information about the {kib} 9.0.0 release, review the following information.
 [[breaking-changes-9.0.0]]
 === Breaking changes
 
+[discrete]
+.Removed legacy alerting endpoints (9.0.0)
+[%collapsible]
+====
+*Details* +
+--
+* `POST /api/alerts/alert/{id?}` has been replaced by `POST /api/alerting/rule/{id?}`
+* `GET /api/alerts/alert/{id}` has been replaced by `GET /api/alerting/rule/{id}`
+* `PUT /api/alerts/alert/{id}` has been replaced by `PUT /api/alerting/rule/rule/{id}`
+* `DELETE: /api/alerts/alert/{id}` has been replaced by `DELETE /api/alerting/rule/{id}`
+* `POST /api/alerts/alert/{id}/_disable` has been replaced by `POST /api/alerting/rule/{id}/_disable`
+* `POST /api/alerts/alert/{id}/_enable` has been replaced by `POST /api/alerting/rule/{id}/_enable`
+* `GET /api/alerts/_find` has been replaced by `GET /api/alerting/rules/_find`
+* `GET /api/alerts/_health` has been replaced by `GET /api/alerting/rule/_health`
+* `GET /api/alerts/list_alert_types` has been replaced by `GET /api/alerting/rule_types`
+* `POST /api/alerts/alert/{alert_id}/alert_instance/{alert_instance_id}/_mute` has been replaced by `POST /api/alerting/rule/{rule_id}/alert/{alert_id}/_mute`
+* `POST /api/alerts/alert/{alert_id}/alert_instance/{alert_instance_id}/_unmute` has been replaced by `POST /api/alerting/rule/{rule_id}/alert/{alert_id}/_unmute`
+* `POST /api/alerts/alert/{id}/_mute_all` has been replaced by `POST /api/alerting/rule/{id}/_mute_all`
+* `POST /api/alerts/alert/{id}/_unmute_all` has been replaced by `POST /api/alerting/rule/{id}/_unmute_all`
+* `POST /api/alerts/alert/{id}/_update_api_key` has been replaced by `POST /api/alerting/rule/{id}/_update_api_key`
+* `GET /api/alerts/{id}/_instance_summary` has been deprecated without replacement. Will be removed in v9.0.0
+* `GET /api/alerts/{id}/state` has been deprecated without replacement. Will be removed in v9.0.0
+--
+
+*Impact* +
+Deprecated endpoints will fail with a 404 status code starting from version 9.0.0
+
+*Action* +
+Remove references to `GET /api/alerts/{id}/_instance_summary` endpoint.
+Remove references to `GET /api/alerts/{id}/state` endpoint.
+Replace references to endpoints listed as deprecated by it's replacement. See `Details` section.
+The updated APIs can be found in {api-kibana}/group/endpoint-alerting
+====
+
+.Removed legacy cases endpoints (9.0.0)
+[%collapsible]
+====
+*Details* +
+--
+* `GET /api/cases/status` has been deprecated with no replacement. Deleted in v9.0.0
+* `GET /api/cases/{case_id}/comments` has been replaced by `GET /api/cases/{case_id}/comments/_find` released in v7.13
+* `GET /api/cases/<case_id>/user_actions` has been replaced by `GET /api/cases/<case_id>/user_actions/_find` released in v8.7
+* `includeComments` parameter in `GET /api/cases/{case_id}` has been deprecated. Use `GET /api/cases/{case_id}/comments/_find` instead, released in v7.13
+--
+
+*Impact* +
+Deprecated endpoints will fail with a 404 status code starting from version 9.0.0
+
+*Action* +
+Remove references to `GET /api/cases/status` endpoint.
+Replace references to deprecated endpoints with the replacements listed in the breaking change details.
+====
+
 [discrete]
 .Removed all security v1 endpoints (9.0.0)
 [%collapsible]

docs/upgrade-notes.asciidoc

@@ -51,7 +51,7 @@ For Elastic Security solution release information, refer to {security-guide}/rel
 
 [discrete]
 [[breaking-201550]]
-.[Alerting] Legacy deprecations (9.0.0)
+.Removed legacy alerting endpoints (9.0.0)
 [%collapsible]
 ====
 *Details* +
@@ -85,7 +85,7 @@ The updated APIs can be found here https://www.elastic.co/docs/api/doc/kibana/v8
 ====
 
 [[breaking-201004]]
-.[Cases] Legacy deprecations (9.0.0)
+.Removed legacy cases endpoints (9.0.0)
 [%collapsible]
 ====
 *Details* +