Add usage billing updated audit event type
What does this MR do and why?
Related to #580868
This MR adds a new audit event type usage_billing_updated that is triggered when a group owner updates the GitLab Credits dashboard settings. This audit event helps track changes to usage billing configuration at the group level. The namespace settings were added initially in Add display_gitlab_credits_user_data to app and... (!214508 - merged) as part of https://gitlab.com/gitlab-org/gitlab/-/issues/580868+.
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Edited by Tyler Amos