Switch to NuGet trusted publishing

[martincostello]

Once we have access (it's still being progressively rolled-out), we should switch from using API keys to publish packages to NuGet.org to the new Trusted Publishing feature.

This is more secure, and avoids the need to renew/rotate API keys. Last time we only found out that the API key had expired when a release failed.

[martincostello]

This should be available now: New Trusted Publishing enhances security on NuGet.org