fix(orders): force string `PurchaseOrderNumber`

[Roman991]

also this field can be edited by customers

[moltar]

@Roman991 thanks for these contribs! Keep them coming 😁

[github-actions[bot]]

:tada: This PR is included in version 1.9.53 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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