Decided to remove the property based approach to specifying a target
framework to just specifying `netcoreapp3.1` directly. The reason for
this is the following:

The advantage of the property is it makes it "easy" to change to a new
target framework in the future. That benefit is actually pretty minimal.
A simple find and replace operation is **extremely** effective in our
code base (it's less key strokes than this message). Hence the benefit
is minimal.

The downside of the property is that our code doesn't look like customer
code. Or rather it diverges from the practices that we publish. In
general I prefer to keep our code as standard as possible unless there
is a good reason to deviate. There just doesn't seem to be one here.

Change Roslyn to target netcoreapp3.1 when building .NET Core assets.

Previously the code targetted a mix of netcoreapp2.1 and netcoreapp3.0.
The mix is due to default interfaces only being supported on
netcoreapp3.0 and hence our testing needed to use that. Yet at the same
time we were required to ship the compiler in SDKS that targetted
netcoreapp2.1. Now we can universally target netcoreapp3.1.

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20077.7

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20076.3

* Update dependencies from https://github.com/dotnet/arcade build 20200124.6

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20074.6

* Update dependencies from https://github.com/dotnet/arcade build 20200125.3

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20075.3

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20073.17

* Update dependencies from https://github.com/dotnet/arcade build 20200122.2

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20072.2

* Update dependencies from https://github.com/dotnet/arcade build 20200122.3

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20072.3

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20071.3

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20068.1

* Update dependencies from https://github.com/dotnet/arcade build 20200116.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20066.1

* Update dependencies from https://github.com/dotnet/arcade build 20200117.2

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20067.2

Use master-vs-deps as ibc source branch as there is no VS branch to insert into (and therefore the binding redirects have the incorrect roslyn version)

* Update dependencies from https://github.com/dotnet/arcade build 20200112.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20062.1

* Update dependencies from https://github.com/dotnet/arcade build 20200113.2

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20063.2

* Update dependencies from https://github.com/dotnet/arcade build 20200115.5

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20065.5

This will cause local Visual Studio and VS Code test runners to run under
CoreCLR by default. This will only affect tests which are multi-targeted,
which today only includes the compiler unit tests.

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20061.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20060.4

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20059.3

* Update dependencies from https://github.com/dotnet/arcade build 20200105.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20055.1

* Update dependencies from https://github.com/dotnet/arcade build 20200106.6

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20056.6

* Update dependencies from https://github.com/dotnet/arcade build 20200107.5

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20057.5

* Update dependencies from https://github.com/dotnet/arcade build 20200108.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20058.1

This reverts commit 27ecd825.

* Rework build definitions to accept VS optprof drop

remove preview 1 publishing

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20054.1

* Update dependencies from https://github.com/dotnet/arcade build 20200102.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20052.1

* Update dependencies from https://github.com/dotnet/arcade build 20200103.2

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20053.2

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20051.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19631.1

* Update dependencies from https://github.com/dotnet/arcade build 20191229.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19629.1

* Update dependencies from https://github.com/dotnet/arcade build 20191230.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19630.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19628.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19627.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19626.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19625.1

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19624.1