Denote Product Analytics as an experimental feature

⚠ This is a breaking change of an experimental feature. Projects that are currently using product analytics will need to enable Experiment Features at the root namespace level after this MR is merged. ⚠

What does this MR do and why?

• Makes it necessary to enable Experiment Features at a root namespace level to use Product Analytics.
• It is still necessary to enable several feature flags too.
• The change is so that when we're ready to move to Beta, we can remove the feature flags but namespace owners still need to opt in specifically to use product analytics.

How to set up and validate locally

• Simulate SaaS in your GDK.
• Make sure product analytics flags are enabled for your GDK. (product_analytics_dashboards, combined_analytics_dashboards)
• Enable experimental features for a root group.
• Assert that:
  • In the root namespace's settings, there is an "Analytics" section and within that is this: - Check that it disappears when you disable Experiment Features.
  • Projects within the namespace have access to the Analytics dashboards link in the Analyze section of the sidebar. When Experiment features are disabled, the link should not be there.

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.

Related to #414865 (closed)