Add severity field to the cost checklist

The existing [cost checklist](https://github.com/Azure/review-checklists/blob/main/checklists/cost_checklist.en.json) is missing the severity field for the recommendations. Without the severity field (which is required according to the [JSON schema](https://github.com/Azure/review-checklists/blob/main/checklists/checklist.schema.json)) the cost recommendations will not load properly in the upcoming web UI 2.0

@brmoreir or @pea-ms please let me know if you would like assistance adding the severity to our existing recommendations as part of our work together on cost management.

Compare checklists/cost_checklist.en.json with checklists/checklist.schema.json to identify the required severity field and review the existing recommendations. Add severity values to the recommendations, then verify that the checklist conforms to the schema and can load in the web UI.

Written by the indexing model from the issue text.