Define PackageProjectUrl which is the public facing property to specify
a project url in a nuspec. Arcade defaults PackageProjectUrl to
RepositoryUrl if it isn't set. RepositoryUrl was incorrectly set to
git://github.com/dotnet/runtime instead of
https://github.com/dotnet/runtime which cause PackageProjectUrl to use
that incorrect value as well which ultimately results in nuget.org
failing package validation and refusing to add the package.

Also changing ProjectUrl to the previous https://dot.net value which is
the expected url to the dotnet project. In addition to that, specifying
Owners which aren't set anymore in 6.0.

Adding the ProjectUrl property as well as the
Microsoft.DotNet.Build.Tasks.Packaging package depends on it. That line
can be removed after the fix to not override its value unconditionally
is merged and consumed.