fix(administration): reset the canonical product of duplicated variants - #21292

1. Why is this change necessary?

When a product is duplicated in the Administration, a variant that stores its own canonical product keeps it, so the copy's variant page names the original product's variant as its canonical URL. Search engines are then told that the copy is a duplicate of the original. #13292 resets canonicalProductId when the parent is cloned, but the variants are cloned one by one afterwards and only get a new parentId and product number.

(The author of #13292 really should've thought of that)

2. What does this change do, exactly?

Duplicated variants no longer keep a canonical product of their own. They inherit the canonical of the new parent product, which the duplicate already resets, so no page of the copy points to the original product.

3. Describe each step to reproduce the issue or behaviour.

1. Create a product with two variants, A and B, and set the canonical product of variant A itself to variant B (via the Admin API, e.g. PATCH /api/product/<A> with {"canonicalProductId": "<B>"}).
2. Open the parent in the Administration and use "Save and duplicate".
3. Check the copy's variant A (canonicalProductId in the Admin API, or <link rel="canonical"> on its storefront page).
   • Before: it points to variant B of the original product.
   • After: it is empty, and the page uses its own URL as canonical.

4. Please link to the relevant issues (if any).

#13292

5. Checklist

• I have written tests and verified that they fail without my change
• I have updated developer-facing release notes if this change is relevant for external developers:
  • Add a short entry to RELEASE_INFO-6.<major>.md under “Upcoming” for informational changes, including the consequences of the change and how it affects external developers.
  • Add an UPGRADE section in UPGRADE-6.<next-major>.md for breaking changes (what/why/impact/how to adapt).
  • See the Documenting a Release Process for details.
• I have written or adjusted the documentation and agent skills according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfilled them