Update to bundler v2.3.15

What does this MR do and why?

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

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

This relates to the surprising discovery that bundler v2.2.33 significantly slows boot times: #364373 (closed)

This merge request also works around an issue with the LicenseFinder and bundler v2.2+ gems that for some reason was only exposed with the upgrade to v2.3.15 (possibly a cache issue). bundler v2.2+ and LicenseFinder do not play well together when:

1. There are native gems installed (e.g. nokogiri, grpc, and google-protobuf).
2. Gemfile.lock doesn't list the platform-specific gems that were installed.

A full explanation is here: https://github.com/pivotal/LicenseFinder/issues/828#issuecomment-953359134

To work around the issue, we configure bundler to install gems for the current Ruby platform, which causes Gemfile and Gemfile.lock to be updated with the platform-specific gems. This allows LicenseFinder to run properly. After it finishes, we clean up the mess.

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