Update gitlab-runner-version to 18.11.1 from 17.6.0

[zosopentoolsmain]

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Summary: Updates the pinned GitLab Runner version used by the build environment.

Changes:

• Bumps RUNNER_VERSION in buildenv from 17.6.0 to 18.11.1 so the derived stable tag/URL track the newer upstream release.

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[augmentcode]

buildenv:2 — This is a major version bump (17→18); do we need to validate any other build/patch logic in this repo against GitLab Runner 18.x (e.g., changed tag naming, dependencies, or build flags) to avoid downstream build failures?

Severity: medium

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