Update version in API spec (#31086)

* Update version in API spec

Look like we miss to update the version in API spec in PR #30966
so it showing 2.5.3 in https://airflow.apache.org/docs/apache-airflow/stable/stable-rest-api-ref.html.
maybe we should consider adding a target in breeze to update the version in all required files?
]

airflow/api_connexion/openapi/v1.yaml

@@ -231,7 +231,7 @@ info:
     This means that the server encountered an unexpected condition that prevented it from
     fulfilling the request.
 
-  version: '2.5.3'
+  version: '2.7.0.dev0'
   license:
     name: Apache 2.0
     url: http://www.apache.org/licenses/LICENSE-2.0.html