chore(ci): align nightly_deployment mute window with 06:00 UTC deploys

[AztecBot]

What

The Grafana nightly_deployment alert mute window (spartan/metrics/grafana/alerts/mutetimes.yaml) was set to 04:00–05:00 UTC, but the nightly deploy workflows all run at 06:00 UTC (cron: "0 6 * * *"):

• .github/workflows/deploy-next-net.yml (the route that actually applies the nightly_deployment mute in policies.yaml)
• .github/workflows/deploy-staging-public.yml
• .github/workflows/deploy-staging-internal.yml

So the deploy-time alert churn was never being muted. This moves the mute window to 06:00–08:30 UTC (2.5h) to cover the deploys.

The 2.5h span accounts for:

1. GitHub cron not firing exactly on time
2. the network deploy itself taking a while
3. the network taking ~3 epochs (~38 min each) to form a committee

Also

Added a concise comment to each nightly-deploy workflow's schedule (and to mutetimes.yaml) noting that changing the deploy timing means updating the Grafana mute window too, so the two don't drift again.