Deprecate NUGET_USE_LEGACY_ASSET_TARGET_FALLBACK_DEPENDENCY_RESOLUTION in .NET 11

[nkolev92]

Added in 6.2 first: https://learn.microsoft.com/en-us/nuget/release-notes/nuget-6.2#summary-whats-new-in-62

We said we were gonna remove this in 7.0, but we didn't, so 11.x it is #11564 (comment)

[dotnet-policy-service]

For changes in NuGet functionality (such as NuGet Restore changes), refer to the .NET SDK breaking change guidelines