Update Unity.gitignore

[whisper3zzz]

Reasons for making this change: PlasticSCM is the official version control tool provided by Unity, it is popular for its fast and convenient interactive experience in the editor, and many developers are using it, but it should be recognised that while using PlasticSCM many developers also have the need to upload their projects to Github.

JetBrains Rider is the world's most loved .NET and game dev IDE，more and more developers are using it.

So I've updated the file for the useless cache files generated by these two popular software.

Links to documentation supporting these rule changes: Plastic SCM JetBrains Rider