* Subtarget support for Unity 2021.2+
Allows passing the -standaloneBuildSubtarget parameter via customParameters in order to facilitate dedicated server builds. Since Unity introduced the subtarget option in 2021.2, this parameter only takes effect in supported Unity versions.
* Fix build subtarget compatibility
Certain versions of Unity do not have a value for `NoSubtarget`. Using `default` will be more robust against churn in this area of Unity's API.
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