More maintenance-packages versions

[carlossanlop]

We have more packages coming out of maintenance-packages.

• System.Memory 4.6.1
• System.Net.WebSockets.WebSocketProtocol 5.1.1
• System.Threading.Tasks.Extensions 4.6.1

We had a previous feed with only two packages, and this new feed includes them too.

• System.Runtime.CompilerServices.Unsafe 6.1.1 - The contents are almost the same, only a few diffs in the generated output.
• System.Reflection.DispatchProxy 4.8.1 - No diffs after running generate.sh against it again.

Looks like I no longer need to modify eng/Build.props, that requirement was removed in #1185 , thanks @ViktorHofer

cc @MichaelSimons @ericstj