Legacy: address validation fix

[garmashdev]

Description

Legacy flow: the plugin address required fields validation is not consistent with Bolt’s requirements. For US address the region field is required, see validator class. Without it it leads to the billing.region must be populated error see DDOG

Fixes: https://app.asana.com/0/1200879031426307/1207408962696795/f

#changelog Legacy: address validation fix

Type of change

• [x] Bug fix (change which fixes an issue)
• [ ] New feature (change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please validate that you have tested your change in at least one of the following areas:

• [x] Successfully tested locally (or docker image)
• [ ] Successfully tested on a staging or sandbox server
• [ ] Successfully tested on a merchant's staging server

For PR Reviewer

• [ ] Reviewed unit tests to make sure we are using real components rather than mocks as much as possible?
• [ ] For any major change (observer, new Bolt feature, core Magento interaction) we must add a feature switch, did you verify this?

Checklist:

• [ ] My code follows the style guidelines of this project.
• [ ] I have performed a self-review of my own code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] New and existing unit tests pass locally with my changes.
• [ ] I have created or modified unit tests to sufficiently cover my changes.
• [ ] I have added my ticket link and provided a changelog message.