Datastore: Add `unverify_user` Method

[Samk13]

:heart: Thank you for your contribution!

Description

• Provides a programmatic way to revert user verification.
• Avoids the need for direct database modifications.
• Enables correction of verification errors from the admin panel.
• partially closes https://github.com/inveniosoftware/invenio-administration/issues/202

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• [x] I'm aware of the code of conduct.
• [x] I've created logical separate commits and followed the commit message format.
• [x] I've added relevant test cases.
• [x] I've added relevant documentation.
• [x] I've marked translation strings.
• [x] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [x] I've NOT included third-party code (copy/pasted source code or new dependencies).
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Frontend

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[ntarocco]

I think that @alejandromumo worked on this ☺️