Set gzip default based on code reloading in endpoint (#5972)

Adjust the default for the gzip flag in the generated endpoint to take
code reloading into account.

Fixes #5960

installer/templates/phx_web/endpoint.ex

@@ -17,12 +17,13 @@ defmodule <%= @endpoint_module %> do
 
   # Serve at "/" the static files from "priv/static" directory.
   #
-  # You should set gzip to true if you are running phx.digest
-  # when deploying your static files in production.
+  # When code reloading is disabled (e.g., in production),
+  # the `gzip` option is enabled to serve compressed
+  # static files generated by running `phx.digest`.
   plug Plug.Static,
     at: "/",
     from: :<%= @web_app_name %>,
-    gzip: false,
+    gzip: not code_reloading?,
     only: <%= @web_namespace %>.static_paths()
 
   # Code reloading can be explicitly enabled under the