Update auditor_billing_page_access flag default to be true

What does this MR do and why?

Relates to [Feature flag] Cleanup `auditor_billing_page_ac... (customers-gitlab-com#6640 - closed)

Updates documented default value of auditor_billing_page_access to true in preparation for complete FF removal.

Screenshots or screen recordings

How to set up and validate locally

In #chat-ops-test, run /chatops run feature get auditor_billing_page_access

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.