Convert timestamps to utc in license usage export

What does this MR do and why?

Part of https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3948

Prior to this change, the timestamps included in the license usage file that can be exported on the subscription page did not include any timezone information. This change will ensure that the included timestamps are converted to utc and that the timezone is included in the license usage export.

How to set up and validate locally

1. You need a license and a few entries for HistoricalData to test this.
2. Go to /admin/subscription and download the license usage file via the Export license usage file button in the top right corner.
3. Confirm the timestamps (generated at and historical data timestamps) include the utc timezone in the license usage file.

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.