Replace thumbnailUrl with logo

[xuhdev]

thumbnailUrl is not part of Organization: https://schema.org/Organization

What does this PR change? What problem does it solve?

The default publisher type, as defined by this theme, is "Organization". However, based on its definition at https://schema.org/Organization, thumbnail is not part of "Organization".

This PR addresses this issue by replacing thumbnailUrl with logo.

Alternatively, if the maintainer feels like it, we can use image as well. The advantage of image is that it is also defined as part of Person: https://schema.org/Person

Was the change discussed in an issue or in the Discussions before?

No.

PR Checklist

• [ ] This change adds/updates translations and I have used the template present here.
• [x] I have enabled maintainer edits for this PR.
• [x] I have verified that the code works as described/as intended.
• [ ] This change adds a Social Icon which has a permissive license to use it.
• [x] This change does not include any CDN resources/links.
• [x] This change does not include any unrelated scripts such as bash and python scripts.
• [ ] This change updates the overridden internal templates from HUGO's repository.