Update Unity gitignore to ignore personal layout changes

[smitdylan2001]

Unity 2021+ has a folder called Layouts (with default-2021.dwlt file for info) which stores all layout changes. This should not be pushed to git

Reasons for making this change: New layout folder/file are user generated files storing the Unity window layout. This is different per person and will be edited every layout change. This should not be included in a repo because of the per-user based nature of the layout. This is added in Unity 2021, which now has an LTS release, so it will be used often now for new projects