Update glab-version to 1.91.0 from 1.57.0

[zosopentoolsmain]

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[augmentcode]

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Summary: Updates the pinned GLab (GitLab CLI) stable build version from 1.57.0 to 1.91.0.
Changes: Bumps GITLAB_CLI_VERSION in buildenv, which updates the stable git tag used during the port build.

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[augmentcode]

GITLAB_CLI_VERSION is now 1.91.0, but upstream v1.91.0 declares go 1.26.1 in its go.mod; if the port/CI toolchain is still on Go 1.20 (e.g., go_120), this bump may make builds fail due to an unsupported Go version.

Severity: medium

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