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

[13xforever]

.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