Documentation and release notes for .NET 11 support

[gfraiteur]

Every user-facing change in the .NET 11 work needs a file under ReleaseNotes/.

• Announce net11.0 support.
• If net8.0 and net9.0 are removed from the set of frameworks PostSharp supports weaving, that is
  a breaking change and must be documented as one.
• Document the state of runtime async support, including which aspects are affected and how to
  disable the feature in a project.

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

— Claude for Gael