Add reason to migration.post_feature_upgrade APi (#4233) (#4365)

(cherry picked from commit 7a7e2a2)

Co-authored-by: Quentin Pradet <quentin.pradet@elastic.co>
Co-authored-by: Laura Trotta <153528055+l-trotta@users.noreply.github.com>

Author: 3 people

output/openapi/elasticsearch-openapi.json

@@ -20,7 +20,8 @@
 export class Response {
   body: {
     accepted: boolean
-    features: MigrationFeature[]
+    features?: MigrationFeature[]
+    reason?: string
   }
 }