.Net: Consolidate Planners namespaces into Planning (#3564)

Contributes to https://github.com/microsoft/semantic-kernel/issues/3151

There's currently both Microsoft.SemanticKernel.Planning and
Microsoft.SemanticKernel.Planners. It's confusing for there to be a
distinction between the two, especially since you need the former in
order to do anything meaningful with the latter. This just consolidates
the latter into the former. One less namespace to reason about.

dotnet/samples/KernelSyntaxExamples/Example66_FunctionCallingStepwisePlanner.cs

@@ -3,7 +3,7 @@
 using System;
 using System.Threading.Tasks;
 using Microsoft.SemanticKernel;
-using Microsoft.SemanticKernel.Planners;
+using Microsoft.SemanticKernel.Planning;
 using Microsoft.SemanticKernel.Plugins.Core;
 using Plugins;
 using RepoUtils;