Build fails unless .NET Core 2.1 is installed

[rdeago]

Description

Version: 1.2.1

ExportAnnotations requires .NET Core 2.1. If only .NET Core 3.1 (or .NET 5) is installed on a machine, as it is common on CI servers, builds will fail.

It happens with this PR, for example: Tenacom/Cecil.XmlDocNames#11

Configuration

Any machine without .NET Core 2.1 (or 2.2, although this one is out of support) will behave as described above.

Regression?

Not a regression.

Other information

We should probably take a hint from Microsoft.Build.Artifacts as to which TFMs to support and how to choose the right binary to use in a build.