Support sorting product by update date

[jxltom]

Current the products can only be sorted by name and price, it will be nice to sort them by update date.

This PR adds this support which will sort products by update_at field of product model.

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

Merging #3470 into master will decrease coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #3470      +/-   ##
==========================================
- Coverage   89.75%   89.75%   -0.01%     
==========================================
  Files         237      237              
  Lines       12943    12947       +4     
  Branches     1312     1314       +2     
==========================================
+ Hits        11617    11620       +3     
  Misses        922      922              
- Partials      404      405       +1

Impacted Files	Coverage Δ
saleor/graphql/product/types.py	97% <80%> (-0.3%)	⬇️

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