Upgrade to System.CommandLine 2.0.0 beta4

[mthalman]

The update-dependencies project is currently implemented on beta1 of System.CommandLine. It should be updated to beta4 to remain current. There are breaking changes between these versions that need to be accounted for.

Current docs are available at https://docs.microsoft.com/dotnet/standard/commandline/.

[lbussell]

Migrated to 2.0.0 beta5 in #1251.