Change state value for unplugged linecard

[ghost]

Please give a short description what your pull request is for:

I updated the line-card state to change the state from "unknown" to "warning." This change aims to draw attention to potential issues when a line-card transitions from an active/working state to "Unplugged." This state should indicate an unexpected removal of the card or a failure in detecting it. However, if the slot is empty without a line-card inserted, it will be labeled as "Empty" without triggering a warning.

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[librenms-bot]

This pull request has been mentioned on LibreNMS Community. There might be relevant details there:

https://community.librenms.org/t/23-7-0-changelog/21841/1