fix: replace scenes in build index with full scene path hash value (#1256)

Migrate from Scenes in Build Index to hash value generated from full scene path.
Removing NetworkSceneManager.GetBuildIndexFromSceneName as it just translated to SceneUtility.GetBuildIndexByScenePath.
Replaced references in NetworkSceneManagerTests with SceneUtility.GetBuildIndexByScenePath.

Author: NoelStephensUnity