Support ASP.NET Core 11

[martincostello]

• Add support for .NET 11.
• Drop support for .NET 8 and 9.

TODO

Before merge:

• Use stable version of .NET 11 SDK
• Use stable versions of Microsoft.AspNetCore NuGet packages
• Use Microsoft.OpenApi 3.x.x for net11.0 ()
• Use Microsoft.OpenApi 3.x.x for all TFMs ()
• Support OpenAPI 3.2
• Remove NU5104 suppression
• Check VersionPrefix is correct for the next major version at the time of merge ()
• Remove any members decorated with [Obsolete] that are appropriate (code search)
• Update documentation samples
• Create migration guide

---

Prerelease builds from this PR can be consumed from MyGet:

<PackageReference Include="Swashbuckle.AspNetCore" Version="11.0.0-pr.3786.*" />

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <clear />
    <add key="domaindrivendev" value="https://www.myget.org/F/domaindrivendev/api/v3/index.json" />
    <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
  </packageSources>
</configuration>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.00%. Comparing base (0c48ff0) to head (f10e03b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3786      +/-   ##
==========================================
- Coverage   95.04%   95.00%   -0.05%     
==========================================
  Files         111      111              
  Lines        3958     3965       +7     
  Branches      801      802       +1     
==========================================
+ Hits         3762     3767       +5     
- Misses        196      198       +2     

Flag	Coverage Δ
Linux	95.00% <100.00%> (-0.05%)	⬇️
Windows	95.00% <100.00%> (-0.05%)	⬇️
macOS	95.00% <100.00%> (-0.05%)	⬇️

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[martincostello]

I'd been intentionally avoiding updating the version for now to avoid needing to resolve merge conflicts every time there's a new release from the default branch between now and November. The version that was in the PR description was just copy-paste I hadn't updated correctly - it should now be correct enough as of the time of writing.

There's no guarantee this will be v11 when it eventually gets merged - v10 supporting ASP.NET Core 10 was just a coincidence that the version numbers matched. For example if OpenAPI 3.2 support (#3646) gets merged ahead of .NET 11, then the release for this PR would be at least v12.

I'll update the version for this PR and the snippet in the PR description once the next patch release ships or I update this PR for 11.0.0-preview.2, whichever comes first.

I'm not planning on restoring the .NET 8 and 9 TFMs as they'll be out-of-support by the time this gets merged, and keeping them is just more maintenance burden for me and goes against the intention of this PR to effectively be a preview of what you'll get in November 2026.

[martincostello]

The CI for this PR is now tracking a major version of 11.

[bkoelman]

There's no guarantee this will be v11 when it eventually gets merged

If you're changing major versions over time and only updating the PR description, consumers have no way to track it. In that case, the recommended version in the PackageReference should just be: *-pr.3786.*.

Spam.