Added scenario for changing gitlab.com plan

Why is this change being made?

To add a use case for billing requiring a plan to be downgraded due to non-payment. This scenario requires the person to open an issue and use the change plan request template.

I didn't want to edit the similar scenarios, so I just created a new one specifically for billing.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what