[ci] Disable .NET jobs by default on d17-5 (#8230)

Disables all of the jobs that are unique to .NET Android NuGet package
creation, testing, and signing, as we we do not plan to release new
nuget packages from the `d17-5` branch.

This behavior is controlled by a new pipeline parameter named
`createSignNupkgs` which can be set to `true` if it's ever needed.