Make OnJoin private

[endel]

The room's OnJoin event is internal and should not be exposed publicly:

colyseus-unity-sdk/Assets/Colyseus/Runtime/Colyseus/Room/ColyseusRoom.cs

Lines 193 to 196 in 47ff44a

	/// <summary>
	/// Occurs when the <see cref="ColyseusClient" /> successfully connects to the <see cref="ColyseusRoom{T}" />.
	/// </summary>
	public event ColyseusOpenEventHandler OnJoin;

This causes confusion to some users starting out as OnJoin is not necessary to detect joins since version 0.14. This event is leftover from previous iterations and must be refactored.