Automation: Fix explore button issue for prime builds

[yonasberhe23]

Summary

Fixes #rancher/qa-tasks#2188

Use a stable selector for the cluster list Explore button so tests work on both Prime and Community. Target the Explore action with data-testid="cluster-manager-list-explore-management" instead of column index 7, since Prime uses column 8.

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[izaac]

clean