Resolve NU1701 LFMerge build warnings

[megahirt]

Warning 2: NU1701

The other warning that shows up a lot, which we might want to suppress, is warning NU1701, which says "Package 'foo' was restored using '(list of .NET Framework targets such as 4.6.1, 4.6.2, 4.7.1, etc.)' instead of the project target framework 'net6.0'. This package may not be fully compatible with your project." In fact, all the .NET Framework packages we're inheriting are fully compatible with the net6.0 target, because none of them use APIs that are missing from the .NET 6 release. NuGet can't know that, so it prints a warning, but we'll probably want to suppress that warning.

To suppress this one for an individual package, we'd do something like this:

 <PackageReference Include="L10NSharp" Version="5.0.0-beta0094">
     <NoWarn>NU1701</NoWarn>
 </PackageReference>

Or to suppress it for all packages, we could do this:

<PropertyGroup>
    <NoWarn>$(NoWarn);NU1701</NoWarn>
</PropertyGroup>

This issue was original reported over at #285 and I split that issue into two (#285 and #303)

[josephmyers]

Having been a grizzled, old resident of UpgradeLand for the past year, I would hesitate to suppress NU1701. As long as you know the risks doing so, it's fine, but technically one of our now or future dependencies could make an unsupported call. Suppressing this would of course mean we discover it at runtime instead of compile time. Ideally we don't have these warnings in the first place

^ This was a comment on the original issue

[josephmyers]

Recommended as part of this is to create a shared document that tracks the status of these remaining .NET Framework dependencies. Make sure the proper parties can view progress on resolving these.