Update gem version to 3.28.6

What does this MR do?

Bump the version of the license-management Ruby gem. This is a follow-up MR to !33 (merged) where the version of the gem was NOT updated.

What are the relevant issue numbers?

gitlab-org/gitlab#322120 (closed)

Does this MR meet the acceptance criteria?

• [NA] Changelog entry added; there's already a CHANGELOG entry for this
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
  • Ensure that all of the ENV vars supported are propagated to Docker containers upon docker run in job definitions
• Conforms to the code review guidelines
• Security reports checked/validated by reviewer