Set holt_winters FF to false

What does this MR do and why?

As per #419209 (comment 1554546813) we want to disable the use_holt_winters_forecast_for_deployment_frequency feature flag for the time being.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.