fix(helm): Avoid scraping metrics from gateway pod

[Skaronator]

What this PR does / why we need it:

Rename the nginx port name to avoid beeing scraped by the ServiceMonitor: https://github.com/grafana/loki/blob/91a34868db61f2cf4299d618c2e48885ff0a705e/production/helm/loki/templates/monitoring/servicemonitor.yaml#L32-L33

Which issue(s) this PR fixes: Fixes #13201

Special notes for your reviewer:

Checklist

• [x] Reviewed the CONTRIBUTING.md guide (required)
• [x] Documentation added
• [x] Tests updated
• [x] Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [x] For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• [x] If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

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[Skaronator]

I am not sure if we need to do a major version bump. Technically, it's a breaking change, but I don't think it will affect anyone.