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)

Start from the Airflow UI Admin > Variables export and import flow described in the report, then trace how variable JSON is generated and consumed. The change is done when exporting and importing variables preserves their description field; add or update coverage for that round trip if the relevant tests are found.

Written by the indexing model from the issue text.