Migrate Maestro to `Octokit.Webhooks`

[JamieMagee]

Maestro currently uses the Microsoft.AspNetCore.WebHooks.Receivers.GitHub package to receive and process GitHub webhooks. This package has been deprecated as part of the .NET package maintenance effort^1.

GitHub now has an official Octokit.Webhooks (and Octokit.Webhooks.AspNetCore) package^2 that's actively being supported. In the long-term, Maestro should probably be migrated to use the official upstream package.

[premun]

Thanks for the heads up!

[dkurepa]

PR for this issue https://github.com/dotnet/arcade-services/pull/4111. We're only changing this on PCS, since Maestro will get shut down soon

[dkurepa]

The PR above was merged, finishing this issue