Rename BUNDLER_VERSION to BUNDLER_MINOR_VERSION

What does this MR do?

Omnibus GitLab already defines BUNDLER_VERSION as the full x.y.z version, and as a result when it triggers this child pipeline the wrong CI image gets used and the build fails (sample job failure: https://gitlab.com/gitlab-org/omnibus-gitlab/-/jobs/7047221380). The CI image only specifies the Bundler minor release, such as 2.4 or 2.5.

To avoid using the wrong CI image, we need to rename the variable.

Check-list

• Ensure commits include Changelog: trailer
• Verify the change in all affected pipelines.