Default enable `combined_analytics_dashboards`"

What does this MR do and why?

This MR enables the feature flag by default true. In the milestone 17.0 the feature flag will be cleaned up.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

FF State	Sidebar	Dashboards view
Enabled
Doesn't exist
Disabled

How to set up and validate locally

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

1. Set up product analytics
2. Remove the feature flag record if you have any

 Feature.remove(:combined_analytics_dashboards)

1. You should still be able to see product analytics button in the sidebar
2. You should still be able to view the dashboards at /namespace/project/-/analytics/dashboards
3. Disable the feature flag record

 Feature.disabled(:combined_analytics_dashboards)

1. You should not see the product analytics button
2. You should get a 404 at /namespace/project/-/analytics/dashboards

Related to #389067 (closed)