[build] In the release workflow, automatically generate an announceme…

…nt discussion

Adopting new API introduced in octokit/octokit.net#2842

build/BenchmarkDotNet.Build/Runners/ReleaseRunner.cs

@@ -143,6 +143,7 @@ private void PublishGitHubRelease()
                 Draft = false,
                 Prerelease = false,
                 GenerateReleaseNotes = false,
+                DiscussionCategoryName = "Announcements",
                 Body = notes
             }).Wait();
             context.Information("  Success");