Use actual activated at value for license type

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/327226

Depends on !57382 (merged) base branch needs to be changed to master after dependency was merged.

Prior to this change, the value for activated_at in the GraphQL Types::Admin::CloudLicenses::LicenseType class was using created_at since the field didn't exist before. Now that the field exists (see this merge request) in the Gitlab::License class we can use the actual value and implement a fallback to created_at for older licenses.

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