Move nuget signing from client secret towards managed identity

`dotnet sign` is moving away from client secrets and towards managed identity. No further client secrets will be issued and `coverlet`s will expire in approx. 6 months. To sign new releases after that, we need to change our process [release-process](https://github.com/coverlet-coverage/coverlet/blob/master/Documentation/ReleasePlan.md#how-to-manually-release-packages-to-nugetorg).