Support more than one true up SKU

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### Problem to solve

As part of the RevPro implementation, multiple true-up SKUs will be added (https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2098).

During the discussion `@rhardarson` noted:

> I actually think we might need some code changes to make those live, as all true-up roads lead to the singular True-up SKU currently. That's probably worth a dedicated issue.


### Intended users

Unsure.

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### Proposal

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We'll need to modify whereever we handle true up plans to support the possibility of there being multiple, for example:
https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/78c7c5ab8bca8e84ff31197c1ce45a989057f36e/app/services/amend_subscription_service.rb#L121

### Further details

I'm unfamiliar with how this process works, so at the moment I don't have further details to add, but perhaps someone else can contribute.
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Story points: 2.0