Update dependency registry1.dso.mil/ironbank/gitlab/gitlab-runner/gitlab-runner-helper to v15.6.0

This MR contains the following updates:

Package	Update	Change
registry1.dso.mil/ironbank/gitlab/gitlab-runner/gitlab-runner-helper	minor	v15.4.0 -> v15.6.0

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This MR has been generated by Renovate Bot.

Relates https://repo1.dso.mil/platform-one/big-bang/apps/developer-tools/gitlab-runner/-/issues/48