This updates all projects to target .NET Framework 4.7.1. The
TencentCOS and Tardigrade backends depend on .NET Standard 2.0. When a
.NET Framework prior to 4.7.1 is targeted, the system cannot be sure
that all the dependencies exist, so it copies all dependent assemblies
to the output directory. This causes many assemblies from the System
namespace to become bundled in the release.
https://stackoverflow.com/a/48875007
We had previously attempted to make individual projects target 4.7.1
(see pull request #4242), but this can cause compatibility issues when
4.6.2 projects depend on 4.7.1. projects.
This will require Mono 5.10.0 or greater (previously, we required 5.0.0
or greater).
https://www.mono-project.com/docs/about-mono/releases/5.10.0/#class-libraries
This fixes issue #4234.
Kenneth Hsu