Drop the target frameworks announced in SupportPolicy.md (net7.0, net6.0, net5.0, netcoreapp3.1) #1032

SupportPolicy.md announces that Funcky 4 removes the target frameworks that are out of support:

• net7.0
• net6.0
• net5.0
• netcoreapp3.1

Decided at the 4.0.0 grooming on 29 Oct 2026: do this right after the .NET 11 step (#1031), as the first mechanical breaking change, because every later 4.0.0 PR gets smaller once the nine PublicAPI folders of Funcky become five.

Scope

• Funcky: TargetFrameworks become net11.0;net10.0;net9.0;net8.0;netstandard2.1;netstandard2.0. netstandard2.0 and 2.1 stay for .NET Framework 4.8 consumers.
• Funcky.Async: drop net5.0, keep netstandard2.0/netstandard2.1 and the forwarding assets.
• Funcky.Xunit / Funcky.Xunit.v3: net6.0 asset becomes net8.0 (the STACK_TRACE_HIDDEN_SUPPORTED define moves with it).
• Delete the PublicAPI/<tfm> folders of the removed frameworks and the FrameworkFeatureConstants.props groups that no remaining framework needs; constants that are now true everywhere can be removed together with their #if.
• Test projects: the trimming test keeps net8.0 as its oldest framework.
• SupportPolicy.md: move the list from "Next removal" to the done state and name the next candidates (net8.0 leaves support in Nov 2026, net9.0 in May 2026 per the .NET lifecycle, which the policy should address).
• Changelog entry under the 4.0.0 breaking changes.

Part of the 4.0.0 milestone; no dependency on the type redesign.