Werk #16090: etherbox_voltage: No default thresholds and remove predictive levels option

The check plugin etherbox_voltage monitors voltage sensors of Etherboxes. By default, the plugin reported CRIT for any non-zero voltage reading. As of this werk, the new default behaviour is to apply no thresholds. Users can configure custom thresholds, as before.

Furthermore, this werk removes the option to configure predictive levels for this check plugin, which lead to a crash. Already existing configurations will automatically be migrated to the new default behaviour (no thresholds).

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