Update bundler to v2.3.15

What does this Merge Request do and why?

This is just to minimize the versions of bundler used for development. We already use v2.3.6 for development; v2.3.15 will be used everywhere. The GDK runs support/bundle-install in this directory to obtain the version of bundler needed.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.