Drop netstandard support

[martincostello]

• Drop support for netstandard2.0.
• Bump version to 9.0.0.
• Update all NuGet packages to their latest versions where possible.
• Remove redundant NuGet package references.
• Simplify code.
• Remove public members annotated with [Obsolete].
• Remove deprecated --serializeasv2 option.

Resolves #3396.

Will leave this in draft until at least the week of the 2nd June in case there are any issues that come in in the next week from the 8.1.x release that need quick patching (like #3424).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.28%. Comparing base (fca506a) to head (c0e0c6c).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3422      +/-   ##
==========================================
+ Coverage   93.55%   94.28%   +0.73%     
==========================================
  Files         110      109       -1     
  Lines        3833     3728     -105     
  Branches      715      707       -8     
==========================================
- Hits         3586     3515      -71     
+ Misses        247      213      -34     

Flag	Coverage Δ
Linux	94.28% <100.00%> (+0.73%)	⬆️
Windows	94.28% <100.00%> (+0.73%)	⬆️
macOS	94.28% <100.00%> (+0.73%)	⬆️

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[Copilot]

Pull Request Overview

This PR drops netstandard2.0 support, bumps the major version to 9.0.0, updates and cleans up NuGet package references, and removes obsolete code and conditional compilation blocks.

• Remove netstandard2.0 targets and related #if branches across all projects.
• Switch project files to use $(DefaultTargetFrameworks) and bump VersionPrefix to 9.0.0.
• Update package versions in Directory.Packages.props and remove redundant package references.
• Simplify code by removing deprecated options and obsolete attributes.

Reviewed Changes

Copilot reviewed 109 out of 109 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Swashbuckle.AspNetCore.ReDoc/ReDocMiddleware.cs	Removed netstandard conditional code and streamlined middleware serialization and redirects.
src/Swashbuckle.AspNetCore.Newtonsoft/Swashbuckle.AspNetCore.Newtonsoft.csproj	Switched to $(DefaultTargetFrameworks), removed netstandard2.0 target and redundant refs.
src/Swashbuckle.AspNetCore.Newtonsoft/SchemaGenerator/NewtonsoftDataContractResolver.cs	Simplified enum serialization checks; removed #if blocks around StartsWith char/string.
src/Swashbuckle.AspNetCore.Newtonsoft/DependencyInjection/NewtonsoftServiceCollectionExtensions.cs	Removed obsolete alias import for MvcJsonOptions.
src/Swashbuckle.AspNetCore.Cli/Swashbuckle.AspNetCore.Cli.csproj	Switched to $(DefaultTargetFrameworks), removed netstandard2.0 target.
src/Swashbuckle.AspNetCore.Cli/Program.cs	Dropped deprecated --serializeasv2 flag and related code.
src/Swashbuckle.AspNetCore.ApiTesting/Swashbuckle.AspNetCore.ApiTesting.csproj	Switched to $(DefaultTargetFrameworks), removed netstandard2.0 target.
src/Swashbuckle.AspNetCore.ApiTesting/ApiTestRunnerBase.cs	Removed conditional compilation around status‐code check.
src/Swashbuckle.AspNetCore.ApiTesting.Xunit/Swashbuckle.AspNetCore.ApiTesting.Xunit.csproj	Switched to $(DefaultTargetFrameworks), removed netstandard2.0 target.
src/Swashbuckle.AspNetCore.Annotations/Swashbuckle.AspNetCore.Annotations.csproj	Switched to $(DefaultTargetFrameworks), removed netstandard2.0 target.
src/Swashbuckle.AspNetCore.Annotations/SwaggerSubTypesAttribute.cs	Removed obsolete SwaggerSubTypesAttribute class.
src/Swashbuckle.AspNetCore.Annotations/PublicAPI/netstandard2.0/PublicAPI.Shipped.txt	Removed obsolete public‐API entries for SwaggerSubTypesAttribute.
src/Swashbuckle.AspNetCore.Annotations/PublicAPI/PublicAPI.Shipped.txt	Removed obsolete public‐API entries for SwaggerSubTypesAttribute.
src/Swashbuckle.AspNetCore.Annotations/AnnotationsSwaggerGenOptionsExtensions.cs	Removed legacy subtype selector code and #if blocks, inlined new C#12 collection expression.
src/Swashbuckle.AspNetCore.Annotations/AnnotationsOperationFilter.cs	Removed conditional compilation around endpoint metadata retrieval.
Directory.Packages.props	Bumped multiple package versions and removed deprecated references.
Directory.Build.props	Added <DefaultTargetFrameworks> and updated <VersionPrefix> to 9.0.0.

Comments suppressed due to low confidence (2)

src/Swashbuckle.AspNetCore.Newtonsoft/SchemaGenerator/NewtonsoftDataContractResolver.cs:45

• [nitpick] JsonConverterFunc is invoked multiple times per enum value. Consider computing the JSON representation once (e.g., using a local list) and reusing it for both the initial StartsWith check and the full enumValuesAsJson processing.

var serializeAsString = (enumValues.Length > 0) && JsonConverterFunc(enumValues.GetValue(0)).StartsWith('"');

src/Swashbuckle.AspNetCore.Annotations/AnnotationsSwaggerGenOptionsExtensions.cs:74

• [nitpick] The C#12 collection expression [] may be unfamiliar to some readers. For clarity and wider compatibility, consider using Array.Empty<Type>() to explicitly return an empty array.

return [];