Build against the 3.1.0 AspNetCore

The version of the runtime that projects compile against
is stamped in to the runtimeconfig.json. In an earlier change, the AspNetCoreRef version
was bumped up to 3.1.2 which causes running dotnet dev-serve to fail
if you do not have the runtime installed.

In addition, the template json allows roll-forwards to major version (5.0 versions) if that's the only version
available. We do this with the razor compiler:
https://github.com/dotnet/aspnetcore-tooling/blob/master/src/Razor/src/Microsoft.AspNetCore.Razor.Tools/runtimeconfig.template.json