Remove dora forecasting feature

Remove DORA forecasting feature

What does this MR do and why?

This MR removes the experimental DORA deployment frequency forecasting feature from GitLab. The feature was introduced as an experiment but is being removed to simplify the codebase and focus on core DORA metrics functionality.

The removal includes:

• Forecast toggle and related UI components
• Statistical forecasting model and calculation logic
• Documentation for the forecasting feature
• Associated test files and mock data
• Locale strings for forecasting UI elements

This change addresses technical debt by eliminating unused experimental code and reducing maintenance overhead.

References

Related to #420389

Screenshots or screen recordings

Before	After

How to set up and validate locally

NOTE: Ensure you have disabled the dora_metrics_dashboard feature flag

1. Navigate to a project or group with CI/CD analytics access
2. Go to Analyze > CI/CD analytics
3. Select the Deployment frequency tab
4. Verify the "Show forecast" toggle is no longer present
5. Confirm the deployment frequency chart displays without forecast options
6. Check that no forecast-related alerts or UI elements appear
7. Ensure the chart functionality works as expected without forecasting features

Key Changes

Component	Action
Frontend Components	Removed forecast toggle and related UI elements
Backend Logic	Removed linear regression and forecasting calculations
Documentation	Removed forecasting feature documentation
Tests	Removed forecast-related test suites
Localization	Removed forecast UI strings

MR acceptance checklist

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