Add member limit banner to usage quota seats page

What does this MR do and why?

The purpose of this issue is to implement a banner on the usage quota seats page specifically when the namespace moves from a paid tier (either from a paying subscription or a trial) down to the free tier and the namespace has more than 5 members.

Screenshots or screen recordings

How to set up and validate locally

Set up your environment to simulate SaaS with subscriptions and enable the feature flag. Find or create group with more than 5 members and copy group path:

export GITLAB_SIMULATE_SAAS=1
bin/rails c

Feature.enable(:free_user_cap)
Group.find_by(path: GROUP_PATH).gitlab_subscription.update(trial_ends_on: Date.yesterday)

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

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 https://gitlab.com/gitlab-org/gitlab/-/issues/356207