Deprecate environment variable configuration in favor of CLI flags

Summary

postgres_exporter supports env-var configuration in two ways. We want to move fully to CLI flags, deprecate the env vars, and remove them after 2 releases.

Current state

1. Flags with an Envar() fallback, settable via flag or env var. Kingpin doesn't record which one was used: PG_EXPORTER_WEB_TELEMETRY_PATH, PG_EXPORTER_DISABLE_DEFAULT_METRICS, PG_EXPORTER_AUTO_DISCOVER_DATABASES, PG_EXPORTER_EXTEND_QUERY_PATH, PG_EXPORTER_CONSTANT_LABELS, PG_EXPORTER_EXCLUDE_DATABASES, PG_EXPORTER_INCLUDE_DATABASES, PG_EXPORTER_METRIC_PREFIX, PG_EXPORTER_COLLECTION_TIMEOUT. Each already has a matching CLI flag (several already marked DEPRECATED for other reasons).

2. Env-var-only settings with no CLI flag (exporter/datasource.go:GetDataSources()): DATA_SOURCE_NAME, DATA_SOURCE_USER(_FILE), DATA_SOURCE_PASS(_FILE), DATA_SOURCE_URI(_FILE). This is the Postgres connection config, and there's no flag today for setting it.

3. The prometheus-community/helm-charts postgres-exporter chart sets DATA_SOURCE_NAME, DATA_SOURCE_URI(_FILE), DATA_SOURCE_USER(_FILE), and DATA_SOURCE_PASS(_FILE) directly as container env vars in templates/deployment.yaml, confirmed against the chart's current main branch. It has no equivalent flag-based path and will need updating in that separate repo.

Start by auditing the flag definitions and the environment-variable fallbacks listed in the issue, then read exporter/datasource.go:GetDataSources() for connection configuration. Inspect the prometheus-community/helm-charts postgres-exporter templates/deployment.yaml separately. Done means the deprecation path is defined for all listed variables, CLI coverage is addressed, and the chart migration is coordinated for removal after two releases.