Rename Devise initializer template

This makes it obvious that this is an ERB template.

Co-authored-by: Jared Norman <jared@super.gd>

Author: senemsoy

lib/generators/solidus/auth/install/install_generator.rb

@@ -14,7 +14,7 @@ class InstallGenerator < Rails::Generators::Base
         source_root "#{__dir__}/templates"
 
         def generate_devise_key
-          template 'config/initializers/devise.rb', 'config/initializers/devise.rb', skip: true
+          template 'config/initializers/devise.rb.erb', 'config/initializers/devise.rb', skip: true
         end
 
         def add_migrations