automation: Update gitlabrunner Docker tag to v15 - autoclosed

This MR contains the following updates:

Package	Update	Change
gitlabrunner	major	v14.10.1 -> v15.7.1

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Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

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🔕 Ignore: Close this MR and you won't be reminded about this update again.

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