Upgrade to bundler v2.3.15

bundler v2.2.33 introduced a significant performance regression (https://github.com/rubygems/rubygems/issues/4275) in boot time. In my GitLab Omnibus instance, upgrading to v2.3.15 decreased the gitlab-rails console boot time from 60 seconds to 45. In addition, any script that runs /opt/gitlab/embedded/bin/irb would also be significantly impacted.

We already use bundler v2.3.6 in development, so there's no reason to use v2.2.x.

bundler v2.2.33 was upgraded in GitLab 14.9 via omnibus-gitlab!5889 (merged).

Relates to gitlab#364373 (closed)

Omnibus MR: omnibus-gitlab!6139 (merged)