Update Unity.gitignore to ignore *.slnx files

[BarionLP]

Reasons for making this change

Unity now defaults to generating a slnx file instead of a sln file

https://docs.unity3d.com/Packages/[email protected]/changelog/CHANGELOG.html

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

[BarionLP]

how do i get a review? do i have to ping someone?