Add *.DotSettings.user for Rider to Unity.gitignore

[mikerochip]

Reasons for making this change

JetBrains has a layered settings system, and user-specific, "personal layer" settings are not in the Unity.gitignore.

Links to documentation supporting these rule changes

According to this doc

The corresponding settings file <SolutionName>.sln.DotSettings.user is saved in the solution folder. You should not add it to your VCS.

Merge and Approval Steps

• [x] Confirm that you've read the contribution guidelines and ensured your PR aligns
• [x] Ensure CI is passing
• [ ] Get a review and Approval from one of the maintainers