[.NET] Issue 4152 - Use MsBuild variables for Directory.Package.Props

[timparka]

Why are these changes needed?

Added variables for version numbers so its easier to maintain Directory.Package.props

Related issue number

Closes #4152

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[LittleLittleCloud]

Thanks @timparka