update automerge workflow to handle multiple PRs

[pramodbindal]

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[openshift-ci]

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[Copilot]

Pull request overview

Updates the auto-merge-upstream GitHub Actions workflow so it can safely pass multiple PR numbers between steps (without breaking GITHUB_OUTPUT parsing).

Changes:

• Convert gh pr list --jq output to a single space-separated string for both pr_list and success_pr_list.
• Add bash xtrace (set -x) to the listing step for additional logging/debugging.

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