Restore netcoreapp3.1 build to the engine package

[CharliePoole]

As discussed in #1180 (comments by @Cylox), the net core 3.1 builds of the engine were removed from the engine package after version 3.14. Since some people are using them and we still build them, they should be restored.

At the same time, we should examine whether the builds should be included in any other packages.

[CharliePoole]

This issue has been resolved in version 3.16.0

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org