Updated line color for linear regression forecast

What does this MR do and why?

Updates the line color of the linear regression forecast in the CI/CD Deployment Frequency chart to match that of the Deployment Frequency series.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/407849

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Feature.enable(:dora_charts_forecast, GROUP_ID)
• Browse to http://{local-instance}/groups/gitlab-org/-/analytics/ci_cd?tab=deployment-frequency
• Click the toggle
• Verify that the linear regression forecast is using the correct line color

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.