ci: refresh BattleGrid pnpm setup action

[saagpatel]

What

• Updates BattleGrid workflow pnpm setup steps from v4 to v6.
• Covers both main CI and quality-gates workflows.

Why

• GitHub is warning that the older pnpm setup action still runs on Node.js 20.
• Refreshing it keeps public CI ahead of the runtime cutoff.

How

• Replaced pnpm/action-setup@v4 with pnpm/action-setup@v6.

Testing

• ruby YAML parse for updated workflow files.

Performance Impact

• None expected.

Risk / Notes

• PR checks should confirm the newer setup action behaves the same way.