Add package validation for the MinClientVersion attribute

[jeffhandley]

There are quite a few packages in the gallery that have the MinClientVersion property set to the package's version number instead of the NuGet client version.

I suspect this is largely due to bad UI in NuGet Package Explorer where it doesn't state what the Minimum Client Version is for. I've reported that to @dotnetjunky too.

We should add some validation to the gallery around this value though. The best solution would be to find the highest version of the NuGet.Core package in the gallery, then allow the MinClientVersion to be anything up to that version. Anything higher than NuGet.Core's highest version would be an error. Alternatively, we could use the NuGet.Core version the Gallery is built against. Last resort, it's hard-coded.

[jeffhandley]

Examples affected:

1. http://www.nuget.org/packages/AspNetPager/
2. http://www.nuget.org/packages/AspNetPager_CN/