Move nuget signing from client secret towards managed identity

[daveMueller]

dotnet sign is moving away from client secrets and towards managed identity. No further client secrets will be issued and coverlets will expire in approx. 6 months. To sign new releases after that, we need to change our process release-process.

[Bertk]

Additional topics beyond the release process documentation update:

• The Artifact Signing Azure DevOps task should be integrated into the Coverlet nightly build pipeline to ensure that generated artifacts are signed consistently.
• Add a benchmarking step to the release process and compare the results against the previous release, as documented in BenchmarkHistory.md. To ensure meaningful comparisons, benchmarks should always be executed on the same dedicated hardware.

https://learn.microsoft.com/en-us/nuget/nuget-org/trusted-publishing