Upgrade target framework and scripts from .net core 3.1 to .net 6.0

.NET 6.0 is the latest LTS release of .NET, and .NET Core 3.1 will reach the end of support this October.

The method of bundling everything in a single-file executable has changed between 3.1 and 6.0, in particular Windows executable is not a single file anymore
Trimming of assemblies during single-file bundling is much more complicated and not recommended anymore, so I have disabled this feature (this means slightly increased file size of releases)
Inclecate usage instructions in README.md should probably be changed, as it was using native Linux binary since migration to .net core 3.1

inkle / ink