Prefer range operators over Substring()

This syntax is shorter and more intuitive. It also alleviates IDE0057. This can only be done in the .NET 3.x projects; unfortunately, it isn't supported by .NET Standard 2.0, so we can't apply this elsewhere until we do away with .NET Framework 4.8 support in OnTopic 5.0.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/TopicViewResultExecutor.cs

@@ -122,7 +122,7 @@ public ViewEngineResult FindView(ActionContext actionContext, TopicViewResult vi
             // Get content-type after the slash and replace '+' characters in the content-type to '-' for view file encoding
             // purposes
             var acceptHeader = splitHeaders[i]
-              .Substring(splitHeaders[i].IndexOf("/", StringComparison.InvariantCulture) + 1)
+              [(splitHeaders[i].IndexOf("/", StringComparison.InvariantCulture) + 1)..]
               .Replace("+", "-", StringComparison.InvariantCulture);
             // Validate against available views; if content-type represents a valid view, stop validation
             if (acceptHeader is not null) {