[ci skip] Remove final mentions of `static_cache_control` in docs.

It was deprecated in https://github.com/rails/rails/pull/19135.

We're now favoring `public_file_server.headers`.

guides/source/asset_pipeline.md

@@ -1029,7 +1029,9 @@ value is set to seconds with a maximum possible value of `31536000` which is one
 year. You can do this in your rails application by setting
 
 ```
-config.static_cache_control = "public, max-age=31536000"
+config.public_file_server.headers = {
+  'Cache-Control' => 'public, max-age=31536000'
+}
 ```
 
 Now when your application serves an asset in production, the CDN will store the

railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt

@@ -15,8 +15,10 @@ Rails.application.configure do
   # Enable/disable caching. By default caching is disabled.
   if Rails.root.join('tmp/caching-dev.txt').exist?
     config.action_controller.perform_caching = true
-    config.static_cache_control = "public, max-age=172800"
     config.cache_store = :memory_store
+    config.public_file_server.headers = {
+      'Cache-Control' => 'public, max-age=172800'
+    }
   else
     config.action_controller.perform_caching = false
     config.cache_store = :null_store