Change AppHost.cs to apphost.cs and update content with SDK directive

Co-authored-by: mitchdenny <[email protected]>

Author: Copilot

src/Aspire.ProjectTemplates/templates/aspire-apphost-singlefile/.template.config/template.json

@@ -68,7 +68,7 @@
     }
   },
   "primaryOutputs": [
-    { "path": "AppHost.cs" }
+    { "path": "apphost.cs" }
   ],
   "postActions": [
     {

src/Aspire.ProjectTemplates/templates/aspire-apphost-singlefile/9.5/AppHost.cs

@@ -0,0 +1,5 @@
+#:sdk Aspire.AppHost.Sdk@aspireVersion
+
+var builder = DistributedApplication.CreateBuilder(args);
+
+builder.Build().Run();