Use Gemfile.lock again for mailroom version for GitLab 15.11+

What does this MR do and why?

In !1592 (merged), we ignored the gitlab-mail_room version in Gemfile.lock. Instead, we pinned the mail_room version in gitlab-org/charts/gitlab!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 (CNG) builds to fail (gitlab-com/gl-infra/production#4053 (closed)).

Now that CNG images are shipping Ruby 3.0.5, and Gemfile.lock has been updated in gitlab!116494 (merged), we can once again use Gemfile.lock for GitLab 15.11.0 and higher.

Relates to gitlab-com/gl-infra/delivery#2231

Author Check-list

• Has documentation been updated? Yes, see gitlab!116858 (merged), gitlab-development-kit!3065 (merged)