Fix default product tax rate in Dashboard 1.0

[pawelzar]

Resolves #2953

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 #3880 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3880   +/-   ##
=======================================
  Coverage   90.67%   90.67%           
=======================================
  Files         263      263           
  Lines       14214    14214           
  Branches     1423     1423           
=======================================
  Hits        12889    12889           
  Misses        908      908           
  Partials      417      417

Impacted Files	Coverage Δ
saleor/dashboard/product/forms.py	91.9% <100%> (ø)	⬆️
saleor/product/models.py	92.52% <100%> (ø)	⬆️

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