Add note: Mngr approval required for Internal request to extend SaaS trial > 30 days

Why is this change being made?

Adding clear direction re requirement for manager approval when requesting SaaS trial extensions past 30 days. Can find no mention of this for SaaS though clearly exists for Self Managed https://about.gitlab.com/handbook/support/license-and-renewals/workflows/self-managed/trials.html. Internal ticket requests for SaaS extensions past 30 days are a semi regular occurrence. Support can not initiate the trials immediately in this case, but respond to the creator requesting management approval.

https://gitlab.slack.com/archives/C018C623KBJ/p1653009027506789

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