Add best practice to Package-Versioning.md

[ericstj]

@JonDouglas @nkolev92

[learn-build-service-prod]

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[aortiz-msft]

NuGet 2.8.x is out of support. Let's remove the whole of line 121.

[ericstj]

It's still important to specify a version. I know PackageReference requires it. I see nuspecs still allow dependencies without a version. I think this can be reworded and avoid the mention of 2.8.x.

[nkolev92]

How about:

Suggested change

	Always specify a version or version range for package dependencies in project files, `packages.config` files, and `.nuspec` files. Without a version or version range, NuGet 2.8.x and earlier chooses the latest available package version when resolving a dependency, whereas NuGet 3.x and later chooses the lowest package version. Specifying a version or version range avoids this uncertainty.
	Always specify a version or version range for package dependencies in project files, `packages.config` files, and `.nuspec` files. Without a version or version range, when resolving a dependency, consistent restore results are not guaranteed.