fix: ConfigMap "grafana-dashboard-loki_thanos_object_storage" is invalid

[fitz123]

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What this PR does / why we need it: Fixes an invalid ConfigMap name by replacing underscores with hyphens in the dashboard filename. The current name loki_thanos_object_storage.json results in a ConfigMap with name grafana-dashboard-loki_thanos_object_storage, which fails Kubernetes validation as it contains underscores. The PR changes the filename to loki-thanos-object-storage.json to ensure the resulting ConfigMap name complies with Kubernetes naming requirements.

Which issue(s) this PR fixes: NA

Special notes for your reviewer: The error occurs because Kubernetes ConfigMap names must follow the RFC 1123 subdomain naming convention, which doesn't allow underscores. This is a simple fix to ensure dashboard deployments succeed.

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[andrein]

We're using this workaround until this is merged:

dashboards+: std.mergePatch(
  lokiMixin.grafanaDashboards,
  {
      "loki-thanos-object-storage.json": lokiMixin.grafanaDashboards["loki_thanos_object_storage.json"],
      "loki_thanos_object_storage.json": null,
  }),