[Task] Make `nameserver` config option platform specific

[gbraad]

On macOS we only have the user networking mode, and likely on Windows we will also drop system networking mode soon-ish. This means the nameserver configuration option is not available. This config only works for system, as it gets overwritten (and otherwise conflict when used: #1879).

Note: For Linux we still offer this, although it was mostly introduced for macOS and Windows usage.

[stale]

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