Adding a note for older Unity versions

As this guide will likely help with reverse engineering, modding and preserving/restoring older Unity games, I think it's important to mention Unity version support or at least acknowledge tools useful for older engine versions. As such, even though UtinyRipper is no longer maintained, it supports Unity 1.x to 2019.x whereas AssetRipper is actively maintained but currently supports Unity 3.5.0 to 2023.3.X. I would therefore suggest adding a note regarding version support under the "Extracting assets" so this information is clearer if anyone needs it.