Fix voucher start date filtering

[dominik-zeglen]

I want to merge this change because it fixes bug causing filtering by start date impossible.

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. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #5133 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #5133   +/-   ##
=======================================
  Coverage   91.13%   91.13%           
=======================================
  Files         261      261           
  Lines       16855    16855           
  Branches     1501     1501           
=======================================
  Hits        15360    15360           
  Misses       1094     1094           
  Partials      401      401

Impacted Files	Coverage Δ
saleor/graphql/discount/filters.py	90% <100%> (ø)	⬆️

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[django-queries]

Here is the report for c2ed862 (mirumee/saleor @ fix/voucher-date-filters)
Base comparison is f22ee17.

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                    	         10	         10	              0
  complete checkout                          	          8	          8	              0
  create checkout                            	         50	         50	             24

# 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                            	         15	         15	              3
  retrieve product attributes                	          9	          9	              0

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product variant bulk create                	         51	         51	              3
  retrieve variant list                      	         15	         15	              6

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0