Prepare Statistics Seats Card for new statistics card component

What does this MR do and why?

This MR is related to https://gitlab.com/gitlab-org/gitlab/-/issues/415698, and is part of a multi-step work (to ease the review work, hopefully).

Description	MR
Introduce new Usage Statistics Component	!125568 (closed) (closed because of delayed review)
Prepare Statistics Seats Card for new statistics card component	👈 you are here
Use Usage Statistics Component in Statistics Seats Card	!125658 (merged)

You can find the exploratory MR here: !125381 (closed)

In this MR we are:

• moving some constants so that they can be more easily reused in the follow-up MRs
• introduce a new getter, to know when the Plan is Free
• updating the description for usage statistics in Free plans

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

This MR usually requires CustomersDot running, but it should be possible to validate it via CustomersDot Staging.

• Make sure to have the following env variables set up, before starting gdk
  • CUSTOMER_PORTAL_URL to https://customers.staging.gitlab.com
  • GITLAB_SIMULATE_SAAS to true
  • GITLAB_LICENSE_MODE to test
• Create a Group with a Free Plan
• Visit /groups/<your-namespace-name>/-/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.