Airflow variable export-import does not include description field

### Apache Airflow version Other Airflow 2 version (please specify below) ### If "Other Airflow 2 version" selected, which one? 2.6.3 ### What happened? On airflow UI, Admin > Variables When we export variables and save the JSON, it does not include description field. So when imported in another environment, description field has to be updated manually. ### What you think should happen instead? Variable Export and Import should include description field. ### How to reproduce airflow UI, go to Admin > Variables Export all variables and save the JSON Check JSON. ### Operating System Windows ### Versions of Apache Airflow Providers 2.6.3 ### Deployment Google Cloud Composer ### Deployment details composer-2.6.0-airflow-2.6.3 ### Anything else? _No response_ ### Are you willing to submit PR? - [X] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)