Downcase cache-control in generated env files

Since 7.1, Rails supports using Rack 3 (and newly generated apps will
get it by default). However, newly generated apps are currently being
generated with configuration that does not follow the Rack 3 SPEC.

Rack 3 required all response headers be downcased, so this commit
downcases the cache-control headers in generated environment files.

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

@@ -24,7 +24,7 @@ Rails.application.configure do
     config.action_controller.perform_caching = true
     config.action_controller.enable_fragment_cache_logging = true
     <%- end -%>
-    config.public_file_server.headers = { "Cache-Control" => "public, max-age=#{2.days.to_i}" }
+    config.public_file_server.headers = { "cache-control" => "public, max-age=#{2.days.to_i}" }
   else
     config.action_controller.perform_caching = false
   end

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

@@ -15,8 +15,8 @@ Rails.application.configure do
   # loading is working properly before deploying your code.
   config.eager_load = ENV["CI"].present?
 
-  # Configure public file server for tests with Cache-Control for performance.
-  config.public_file_server.headers = { "Cache-Control" => "public, max-age=3600" }
+  # Configure public file server for tests with cache-control for performance.
+  config.public_file_server.headers = { "cache-control" => "public, max-age=3600" }
 
   # Show full error reports and disable caching.
   config.consider_all_requests_local = true