Update copilot opt-in message to inform about changes to subscription

Within our opt-in section of the linked accounts page, we allow for people to opt-in to GH copilot. To ensure people are participating with informed consent, we should update the message to indicate that opting in will cancel any active personal Copilot subscription for the account.

Once active, the Enterprise license will replace any personal account subscription entirely, with no need or prompt to switch. Details on this are in the GitHub documentation for copilot: https://docs.github.com/en/billing/concepts/product-billing/github-copilot-licenses

If you have an active Copilot Pro or Copilot Pro+ plan, and are then assigned a seat as part of a Copilot Business or Copilot Enterprise plan, your personal Copilot plan will be automatically canceled. You will receive a prorated refund for any remaining portion of your personal plan's current billing cycle. You will then be able to continue using Copilot according to the policies set by your company.