Track click on explore plans

What does this MR do and why?

This tracks clicks on "Explore plans" on the Usage Quota page when free_user_cap feature flag is enabled.

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/352638 No changelog, because it's behind a feature flag free_user_cap

Screenshots or screen recordings

Set up snowplow	Click on "Explore all plans"	See tracked snowplow event

How to set up and validate locally

1. Set up snowplow (see first screenshot)
2. Start the GDK

   export GITLAB_SIMULATE_SAAS=1
   bin/rails c
   
   ApplicationSetting.first.update(check_namespace_plan: true)
   Feature.enable(:free_user_cap)

3. Go to the Usage Quota page of a Group (Group -> Settings -> Usage Quota)
4. Enable Snowplow Extension
5. Click on "Explore all plans" to trigger the event
6. See the event in the Snowplow Chrome extension (or the HTTP request)
7. Make sure you don't have any ad-blockers/script blockers enabled

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.