Drop gitlab-mail_room gem update in documentation

What does this merge request do and why?

This extra step is no longer needed now that gitlab!116858 (merged) is merged.

We disabled updates to gitlab-mail_room in the gitlab/Gemfile because updating mail_room required an updated net-imap gem. This gem attempted to install the digest gem, but due to a bug present in Ruby 2.7 (https://bugs.ruby-lang.org/issues/17761), the installation failed and caused Cloud Native GitLab builds to fail (gitlab-com/gl-infra/production#4053 (closed)).

We've updated to Ruby 3.0.5 for Cloud Native GitLab images now and have already bumped the net-imap dependency in gitlab!115755 (merged). As a result, GitLab Rails can update the Gemfile again.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.