Unpin mail_room version

What does this MR do?

This commit will allow the Helm Chart to fetch the version specified in the GitLab Rails Gemfile.lock again.

We pinned the mail_room version in !2236 (merged) because updating mail_room required an updated net-imap gem. net-imap 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-org/gitlab!115755 (merged). With gitlab-org/gitlab!116494 (merged), we've now updated GitLab Rails to use mail_room v0.0.23.

Removing this dependency makes it a bit easier to update mail_room since there is one less version to bump. gitlab-org/gitlab!116858 (merged) updates the documentation.

Testing

I needed !3077 (merged) to verify that running scripts/manage_version.rb -a --app-version 15.10.0 on a branch updates the mailroom/Chart.yaml.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.