Use CPVM for NuGet packages

[safesparrow]

Using Central Package Version Management makes it slightly easier to maintain package versions in the repository.
More info: https://devblogs.microsoft.com/nuget/introducing-central-package-management/

When enabled, all PackageReference items must omit the version attribute and their version must be specified in a separate PackageVersion item, typically residing in a shared .props file.

I'm happy to prepare a PR for this if would be a sensible change.