Falcon2 support drop

[p1c2u]

FalconOpenAPIRequestFactory accepts default_when_empty keyword argument.

Backward compatibilities:

• FalconOpenAPIRequestFactory requires to be instantiated

[codecov]

Codecov Report

Merging #353 (1c87469) into master (82738c3) will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   95.71%   96.05%   +0.33%     
==========================================
  Files          81       80       -1     
  Lines        1705     1697       -8     
==========================================
- Hits         1632     1630       -2     
+ Misses         73       67       -6     

Impacted Files	Coverage Δ
openapi_core/contrib/falcon/handlers.py	100.00% <100.00%> (ø)
openapi_core/contrib/falcon/middlewares.py	100.00% <100.00%> (ø)
openapi_core/contrib/falcon/requests.py	100.00% <100.00%> (ø)
openapi_core/contrib/falcon/responses.py	92.30% <100.00%> (-0.55%)	⬇️
openapi_core/exceptions.py	90.74% <0.00%> (+1.85%)	⬆️
openapi_core/unmarshalling/schemas/exceptions.py	96.42% <0.00%> (+3.57%)	⬆️
openapi_core/templating/responses/exceptions.py	100.00% <0.00%> (+10.00%)	⬆️
openapi_core/validation/exceptions.py	100.00% <0.00%> (+12.50%)	⬆️

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