Add minimal quantity of products for voucher

[fowczarek]

I want to merge this change because resolve #4316 and fix #4379

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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.

[django-queries]

Here is the report for f6a298b (mirumee/saleor @ 4316/voucher_min_qunantity_of_products)
Base comparison is d528263.

No differences were found. (click me)

# api.benchmark checkout
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  add billing address to checkout     	         34	         34	             20
  add shipping to checkout            	          7	          7	              0
  checkout payment charge             	         14	         14	              0
  complete checkout                   	          6	          6	              0
  create checkout                     	         43	         43	             21

# api.benchmark homepage
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve main menu                  	          5	          5	              0
  retrieve product list               	          4	          4	              0
  retrieve secondary menu             	          5	          5	              0
  retrieve shop                       	          2	          2	              0

# api.benchmark product
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  product details                     	         13	         13	              3

# api.benchmark variant
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve variant list               	          9	          9	              2

[codecov]

Codecov Report

Merging #4427 into master will increase coverage by 0.04%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##           master    #4427      +/-   ##
==========================================
+ Coverage    90.7%   90.75%   +0.04%     
==========================================
  Files         291      291              
  Lines       16680    16694      +14     
  Branches     1691     1692       +1     
==========================================
+ Hits        15130    15150      +20     
+ Misses       1054     1049       -5     
+ Partials      496      495       -1

Impacted Files	Coverage Δ
saleor/graphql/discount/types.py	89.28% <ø> (ø)	⬆️
saleor/discount/utils.py	96.25% <100%> (+0.19%)	⬆️
saleor/graphql/discount/mutations.py	98.2% <100%> (+0.01%)	⬆️
saleor/dashboard/discount/forms.py	98.05% <100%> (ø)	⬆️
saleor/discount/models.py	84.37% <100%> (+0.9%)	⬆️
saleor/checkout/utils.py	83.72% <100%> (+0.59%)	⬆️
saleor/dashboard/order/utils.py	77.11% <50%> (+0.84%)	⬆️
saleor/payment/utils.py	92.01% <0%> (+0.93%)	⬆️

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