Update deployment frequency graphs to use UTC dates

What does this MR do?

Updates the deployment frequency graphs (originally implemented in !50885 (merged)) to use UTC dates instead of local dates for all calculations and display.

This aligns with the assumptions made by the backend API endpoint - the time interval it sends are UTC days. In addition, this allows the frontend to no longer depend on the specific date format sent by the API.

Some context that triggered this change: !51938 (comment 494476653)

Screenshots

The main user-facing change is that the date range of the charts has shifted slightly - the start date is one day later, and the graph includes data for the current date. (Previously, the current date was shown on the chart, but it always showed 0 deployments).

Before	After

Related to #291747 (closed)