Move postgres-exporter settings to underscored form

Replace the usage of node['monitoring']['postgres-exporter'] with node['monitoring']['postgres_exporter'] as mentioned in #6873 (closed).

Related issues

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/work_items/123352312 under #6873 (closed)

Checklist

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Expected (please provide an explanation if not completing)

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