Add manually-triggered workflow to remove PendingRelease label from all issues

[magnesj]

Adds a new GitHub Actions workflow that removes the PendingRelease label from all issues in the repository.

Details

• Trigger: workflow_dispatch only — runs exclusively when started manually from the Actions tab. No automatic triggers.
• Behavior: Paginates over every issue carrying the PendingRelease label (all states) and removes the label, logging each affected issue and a final count.
• Permissions: Scoped to issues: write, matching the existing close-label.yml workflow and using the same actions/github-script@v9 action.

This complements close-label.yml (which adds PendingRelease to closed issues) by providing a one-click way to clear the label after a release.