fix: in helm chart fix indentation in nginx gateway config template handling

[JimCroftMaltedAI]

What this PR does / why we need it:

This PR fixes the indentation of the nginx config in configmap-gateway.yaml so that it works with both the default nginx gateway config (defined as loki.nginxFile in _helpers.tpl) and any user supplied configs supplied via the helm value gateway.nginxConfig.file.

The current code's handling of indentation means any user supplied gateway.nginxConfig.file is not indented correctly resulting in a helm error:

error converting YAML to JSON: yaml: line 13: did not find expected comment or line break

Which issue(s) this PR fixes:

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• 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
• 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]

All committers have signed the CLA.

[jkroepke]

LGTM