Upgrade to bundler v2.3.15
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.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/364373

Changelog: performance

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab