Update attribute vat_id frontend_label to make it translatable

[JeroenVanLeusden]

Description

VAT Number field in checkout isn't translated because label is VAT number. This PR will update the frontend_label of vat_id to make it translatable again.

Fixed Issues (if relevant)

1. None that I could find.

Manual testing scenarios

1. Install for example https://github.com/ho-nl/magento2-nl_NL (Source of strings is Crowdin).
2. On checkout page VAT Number isn't translated.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

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