Add redis db to the mail_room option hash

What does this MR do and why?

This MR adds redis db to the options sent to mail_room gem.

The problem being solved is described in https://gitlab.com/gitlab-org/gitlab-mail_room/-/merge_requests/33. Basically the db from redis url (redis://url?db=X) is not taken into account for a connection and needs do be specified as a separated option.

This MR:

• changes the mail_room.yml configuration
• adds the db value to the option hash used for mail_room

How to set up and validate locally

One way how to verify this is when trying to use incoming emails functionality.

Prerequisites

• incoming emails configured
• local copy of gitlab-mail_room branch
• local gitlab-mail_room gem specified in gitlab Gemfile gem 'gitlab-mail_room', '~> 0.0.9', require: 'mail_room', path: "PATH_TO_LOCAL_GEM"

Validation

• stop the rails-background-jobs in order to be able to verify gdk stop rails-background-jobs
• run mail_room bundle exec mail_room -c './config/mail_room.yml'
• send an email to the address defined in incoming emails configuration
• check the background jobs on http://127.0.0.1:3000/admin/background_jobs - email_receiver should contain a new job

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.