Relax SDK version pinning in global.json

[ralphhendriks]

As discussed on Gitter, it seems reasonable to relax the .NET Core SDK version pinning a bit. Right now, this repo is tightly pinned to 3.1.102.

See here for configuration options in global.json.

Pinning to the feature band (e.g. 3.1.1xx) seems to give the right trade off between build consistency and developer friendliness.