fix(helm): use correct GEL image tag and set enterprise gateway proxy URLs for SSD

[monodot]

What this PR does / why we need it:
Updates the image tag referenced in the deployment to 3.0.0 (without the v prefix) and adds a set of downstream proxy URLs in the enterprise-gateway that support SSD mode (so that the Ring Health tab in the Grafana Enterprise Logs plugin is able to list the nodes).

Which issue(s) this PR fixes:
Fixes #12845

Also uses the bucket name provided in loki.storage.bucketNames.admin, instead of the hardcoded Admin bucket name admin.

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
• 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
• 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

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