Use thumbnail instead of thumbnailUrl in product resolver

[dominik-zeglen]

Resolves #3458

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0ded2ff). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3484   +/-   ##
=========================================
  Coverage          ?   89.87%           
=========================================
  Files             ?      241           
  Lines             ?    13103           
  Branches          ?     1322           
=========================================
  Hits              ?    11776           
  Misses            ?      922           
  Partials          ?      405

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