Add NotFound response for DELETE methods in OpenAPI YAML (#13550)

NotFound (404) is a valid possible response
for 'Delete a Connection/Dag Run/Variable',

but they were missed in the OpenAPI Doc YAML.

airflow/api_connexion/openapi/v1.yaml

@@ -356,6 +356,8 @@ paths:
           $ref: '#/components/responses/Unauthenticated'
         '403':
           $ref: '#/components/responses/PermissionDenied'
+        '404':
+          $ref: '#/components/responses/NotFound'
 
   /dags:
     get:
@@ -628,6 +630,8 @@ paths:
           $ref: '#/components/responses/Unauthenticated'
         '403':
           $ref: '#/components/responses/PermissionDenied'
+        '404':
+          $ref: '#/components/responses/NotFound'
 
   /eventLogs:
     get:
@@ -1060,6 +1064,8 @@ paths:
           $ref: '#/components/responses/Unauthenticated'
         '403':
           $ref: '#/components/responses/PermissionDenied'
+        '404':
+          $ref: '#/components/responses/NotFound'
 
   /dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries:
     parameters: