Target netstandard2.0 for libraries, target net8 for tests, and switch to github actions for CI

[mdabros]

A first step in an effort to get SharpLearning up to date with tools.

• Use github actions for CI. First iteration of this is without nuget publishing.
• Target net8 for test projects.
• Target netstandard2.0 only for libraries . So removing net461 targeting.

Since BinaryFormatter is deprecated from net7 it was necessary to use EnableUnsafeBinaryFormatterSerialization in the SharpLearning.InputOutput.Test project file. Likewise this will also be necessary for other projects using the GenericBinarySerializer. The GenericBinarySerializer will be removed in a later release since it has been deemed unsafe.

  <PropertyGroup>
    <EnableUnsafeBinaryFormatterSerialization>true</EnableUnsafeBinaryFormatterSerialization>
  </PropertyGroup>

Read more here:
https://learn.microsoft.com/en-us/dotnet/core/compatibility/serialization/7.0/binaryformatter-apis-produce-errors#recommended-action