Upgrade Gemfile.lock to use bundler to v2.3.15

This is just to minimize the versions of bundler used for development.
The GDK runs `support/bundle-install` in this directory to obtain the
version of bundler needed.

This relates to https://gitlab.com/gitlab-org/gitlab/-/issues/364373.

Author: stanhu

Gemfile.lock

@@ -113,4 +113,4 @@ DEPENDENCIES
   rspec (~> 3.8.0)
 
 BUNDLED WITH
-   2.3.6
+   2.3.15