Build PostSharp with the .NET 11 SDK #93

Build PostSharp itself with the .NET 11 SDK. This is independent of supporting net11.0 as a target.

• Check the version pinned in global.json and the SDK list in Root.Defs.props.
• PostSharp's own C# should stay on its current LangVersion until it is raised deliberately. When
  it is raised to 15, the source-level breaking changes listed in the analysis apply. No type in
  Core, Patterns, Utilities, UserInterface or Tests is named union or closed, so that
  particular break is already clear.
• Do not set runtime-async=on in PostSharp's own projects while the runtime async work is open.

Analysis: Documentation/DotNet11-CSharp15-Impact.md in postsharp/PostSharp.

— Claude for Gael