Add net11.0 target / drop net8.0 and net9.0

[mus65]

This adds a net11.0 target and drops net8.0 and net9.0 as they will be EOL when .NET 11 releases.

Notes:

• Coverlet doesn't work with .NET Framework. It seems like Coverlet would need to add a net11.0 target first, see also [BUG] TypeInitializationException when targeting .NET Framework coverlet-coverage/coverlet#1818 .I removed it for now. Not sure if this is worth dealing with?
• Removed Microsoft.Extensions.Logging.Abstractions from net11.0 because it is part of the runtime now (see https://learn.microsoft.com/en-us/dotnet/core/compatibility/extensions/11/extensions-in-shared-framework )
• TODO: use stable SDK in global.json

I could split dropping net8.0 and net9.0 into a separate PR if you want.