Rename app.py to main.py in template (#12628)

Author: eerhardt

src/Aspire.ProjectTemplates/templates/aspire-py-starter/13.0/app/app.py renamed to src/Aspire.ProjectTemplates/templates/aspire-py-starter/13.0/app/main.py

@@ -11,7 +11,7 @@
 var cache = builder.AddRedis("cache");
 
 #endif
-var app = builder.AddUvicornApp("app", "./app", "app:app")
+var app = builder.AddUvicornApp("app", "./app", "main:app")
     .WithUvEnvironment()
     .WithExternalHttpEndpoints()
 #if UseRedisCache