Show tooltip when free user cap applies

What does this MR do and why?

This shows a tooltip on the help link when the free user cap applies for a group.

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/352638

Screenshots or screen recordings

How to set up and validate locally

Start GDK as if it would be SaaS, start rails console

export GITLAB_SIMULATE_SAAS=1
gdk start

bin/rails c

# Set your instance to be on namespace-plans
ApplicationSetting.first.update(check_namespace_plan: true)
Feature.enable(:free_user_cap)

Then go to the group Usage Quota page http://localhost:3000/groups/GROUP_NAME/-/usage_quotas#seats-quota-tab of a free group.

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.