Incorrect use of UNITY_SERVER causes Mirror code to compile without Mirror

[tinh98]

#if (MIRROR || UNITY_SERVER) && !FISHNET

https://github.com/Metater/MetaVoiceChat/blob/main/NetProviders/Mirror/MirrorNetProvider.cs#L1
https://github.com/Metater/MetaVoiceChat/blob/main/NetProviders/Mirror/MirrorFrame.cs#L1

UNITY_SERVER is only a flag for headless server builds, it does not imply Mirror. Using (MIRROR || UNITY_SERVER) wrongly includes Mirror code even if neither Mirror nor FishNet are used (e.g. when using another networking provider), causing compile errors.