Add monkey patch for YAML.safe_load_file for MailRoom in Ruby 2.7

What does this MR do and why?

!117027 (merged) used YAML.safe_load_file, but this method is not available in Ruby 2.7.

!113358 (merged) added a temporary monkey patch that makes this available for Ruby 2.7, but since lib/gitlab/mail_room.rb is loaded independently of the Rails process it does not load the initializer.

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