Dependency management standardization changes

[andystaples]

We should make a couple of changes to this project to global package management and .NET SDK versioning to make developer experience more consistent.

1. Add global.json with a standardized .NET SDK version for local dev + CI
2. Enable transitive pinning in Directory.Packages.props, remove transitive package references from .csproj files, check for dependency changes across the repo
3. Create a Directory.Packages.props file in /test directory and move test deps from the main Directory.Packages.props file to the new one.

[andystaples]

For more details see the discussion on #3233