Make PkgProj permit no VersionSuffix being set

[ericstj]

We should fix the dotnet/runtime packaging infra to not require VersionSuffix. See https://github.com/dotnet/runtime/pull/42872/files#r497644103

We ought to permit an unset VersionSuffix and error any place where we would have needed one to be set (EG: a stable package referencing non-stable package).

[ericstj]

cc @ViktorHofer @Anipik

[AraHaan]

Are you refering to this part of the code in arcade?

https://github.com/dotnet/arcade/blob/main/src/Microsoft.DotNet.Arcade.Sdk/tools/Version.BeforeCommonTargets.targets#L119-L141

If so, I want to change this to where it won't override what the user sets in their eng\Versions.props on their repository if they wanted to manually set the version suffix for some reason after importing the arcade sdk itself.

I was thinking of filing a new issue but then I found this one.

[ericstj]

No, this was for pkgproj. I don't think we need this issue any more as we are moving off of pkgproj.