Use .tt extension to all the template files

Make clear that the files are not to be run for interpreters.

Fixes #23847.
Fixes #30690.
Closes #23878.

railties/lib/rails/generators/rails/assets/templates/stylesheet.css

@@ -1,4 +1,4 @@
-/* 
+/*
   Place all the styles related to the matching controller here.
   They will automatically be included in application.css.
 */

railties/lib/rails/generators/test_unit/job/job_generator.rb

@@ -8,7 +8,7 @@ class JobGenerator < Base # :nodoc:
       check_class_collision suffix: "JobTest"
 
       def create_test_file
-        template "unit_test.rb.erb", File.join("test/jobs", class_path, "#{file_name}_job_test.rb")
+        template "unit_test.rb", File.join("test/jobs", class_path, "#{file_name}_job_test.rb")
       end
     end
   end

railties/test/generators/generators_test_helper.rb

@@ -43,9 +43,9 @@ def self.included(base)
   end
 
   def copy_routes
-    routes = File.expand_path("../../lib/rails/generators/rails/app/templates/config/routes.rb", __dir__)
+    routes = File.expand_path("../../lib/rails/generators/rails/app/templates/config/routes.rb.tt", __dir__)
     destination = File.join(destination_root, "config")
     FileUtils.mkdir_p(destination)
-    FileUtils.cp routes, destination
+    FileUtils.cp routes, File.join(destination, "routes.rb")
   end
 end