Add alert for free plans at limit as a preview

What does this MR do and why?

• adds a preview free use cap feature flag as we want to show some warnings/alerts to users before the cutover date
• adds alert to groups and projects if on free plan, on SaaS, over the limit of 5 and feature flag is enabled.
• full spec in #355782
• snowplow raw events good_events.json

note Changes are behind a feature flag.

Screenshots or screen recordings

mobile	fixed	fluid

How to set up and validate locally

1. Create a new group.
2. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#act-as-saas and restart GDK
3. Feature.enable(:preview_free_user_cap) in console.
4. ApplicationSetting.first.update(check_namespace_plan: true) in console.
5. Set this method to return true with a return true just under the method definition.

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 #355782