Merge branch 'avoid-constant-warnings-in-rails' into 'master'

Rails prefers require_dependency so that it won't require twice:

## What does this MR do?

Fix warnings for loading constants twice

## Are there points in the code the reviewer needs to double check?

Now it's depending on `require_dependency`.

## Why was this MR needed?

Fixes #20724

Backtrace before this patch: $23876

See merge request !5727

lib/gitlab/email/receiver.rb

-require 'gitlab/email/handler'
+require_dependency 'gitlab/email/handler'
 # Inspired in great part by Discourse's Email::Receiver
 module Gitlab

lib/gitlab/request_profiler/middleware.rb

 require 'ruby-prof'
-require 'gitlab/request_profiler'
+require_dependency 'gitlab/request_profiler'
 module Gitlab
   module RequestProfiler