feat: add jobPosting support for 'confidential'

[neilhsmith]

Description of Change(s):

Adds a new "confidential" option to a JobPosting's hiringOrganization. Noted in #956.

Noted in Google's Structured Data docs: "If the organization is hiring anonymously (for example, a staffing agency on behalf of an anonymous employer or an employer directly on your platform), use the confidential value for the HiringOrganization field." - https://developers.google.com/search/docs/appearance/structured-data/job-posting#job-posting-definition

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• Updated the documentation
• Unit/Cypress test covering all cases

Please link to relevant Google Docs or schema.org docs for what you are adding so we can review.

Please have a read of the Contributing Guide for full details.

https://github.com/garmeeh/next-seo/blob/master/CONTRIBUTING.md

[garmeeh]

Thanks @neilhsmith 🙏 I will get this released today.