Add redis-namespace gem for mailroom

What does this MR do and why?

This MR adds redis-namespace gem back to the Gitlab application's Gemfile. This reverts the change in !135344 (comment 1677621172) and also updates the gem to 1.10. The gitlab mailroom gem needs redis-namespace as it is defined as a development dependency (https://gitlab.com/gitlab-org/ruby/gems/gitlab-mail_room/-/blob/578c314fc5d936978fafdea68952f9a5b2f7fe7d/mail_room.gemspec#L36).

See the issue #433493 (comment 1679836628).

Mailroom uses redis-namespace as a development dependency. In gitlab-org/charts/gitlab!3419 (diffs), it "deprecates" the use of namespace by not configuring the mailroom to use namespace for the Sidekiq delivery method. But the mailroom gem still could use namespace for Sidekiq delivery if the configuration is overrode. Furthermore since arbitration could use namespace, redis-namespace is still a dependency for mailroom.

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