config: fix DynamicSmtpSettingsInterceptor warning again

This warning re-appeared when running mailer tests:

```
DISABLE_SPRING=1 bin/rspec spec/mailers/administration_mailer_spec.rb
```

It is now fixed properly, in a way recommanded by the documentation.

config/initializers/dynamic_smtp_settings_interceptor.rb

@@ -1,3 +1,14 @@
+# We want to register an interceptor, but we can't make the action idempotent
+# (because there's no way to peek at the currently registered interceptors).
+#
+# To make zeitwerk happy, instead signal that we don't want the
+# DynamicSmtpSettingsInterceptor constant to be auto-loaded, by:
+# - adding it to a non-autoloaded-path (/lib),
+# - requiring it explicitely.
+#
+# See https://guides.rubyonrails.org/autoloading_and_reloading_constants.html#autoloading-when-the-application-boots
+require 'action_mailer/dynamic_smtp_settings_interceptor'
+
 ActiveSupport.on_load(:action_mailer) do
-  ActionMailer::Base.register_interceptor "DynamicSmtpSettingsInterceptor"
+  ActionMailer::Base.register_interceptor DynamicSmtpSettingsInterceptor
 end