Deprecate SerializeAsV2

[martincostello]

As a path towards OpenAPI v3.1 support (see #3283), deprecate the --serializeasv2/SerializeAsV2 options in favour of a new --openapiversion/OpenApiVersion option that accepts the specification version to use.

This can go out in the same release as #3183.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 90.90909% with 4 lines in your changes missing coverage. Please review.

Project coverage is 84.84%. Comparing base (114c2f0) to head (ef5ab53).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
...SwaggerGen/DependencyInjection/DocumentProvider.cs	66.66%	2 Missing ⚠️
...uckle.AspNetCore.Swagger/SwaggerEndpointOptions.cs	80.00%	1 Missing ⚠️
...c/Swashbuckle.AspNetCore.Swagger/SwaggerOptions.cs	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3286      +/-   ##
==========================================
+ Coverage   83.56%   84.84%   +1.28%     
==========================================
  Files          77       77              
  Lines        3224     3221       -3     
  Branches      559      556       -3     
==========================================
+ Hits         2694     2733      +39     
+ Misses        530      488      -42     

Flag	Coverage Δ
Linux	66.31% <20.45%> (-17.25%)	⬇️
Windows	66.31% <20.45%> (-17.25%)	⬇️
macOS	83.54% <90.90%> (-0.02%)	⬇️

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