Upgrade to bundler v2.3.15

What does this MR do?

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 !5889 (merged).

Related issues

Relates to gitlab#364373 (closed)

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

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 the GitLab Chart opened