Upgrade bundler version to v2.3.15

What does this MR do?

Changes: https://github.com/rubygems/rubygems/blob/master/bundler/CHANGELOG.md

This matches the version that will be used with Omnibus: gitlab-org/omnibus-gitlab!6139 (merged)

Related issues

This relates to the surprising discovery that bundler v2.2.33 significantly slows boot times: gitlab-org/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

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated