feat!: Update the Order schema to make currency a required field

[deinck6]

Description

This PR is making the currency field in Order required. This is a followup change from PR 210. When adding the currency field to the Order schema the intention was to make it required during the next version update / breaking change.

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing
  functionality to not work as expected, including removal of schema files
  or fields)

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Is this a Breaking Change or Removal?

If you checked "Breaking change" above, or if you are removing any schema
files or fields:

• I have added ! to my PR title (e.g., feat!: remove field).
• I have added justification below.

## Breaking Changes / Removal Justification

The total values within the Order capability are only represented by an amount and rely on platforms having context of the currency established during Checkout. Currency has sense been added to the Order schema but as an optional field to avoid a breaking change. The intention was to make this required during the next version update / breaking change. Currency should be a required field because capabilities are meant to function independently of one another and the monetary values contained within Order are incomplete without this context.