Switch Reflex project to unity 6000.0.67f1

[octia]

Description

Current project (in unity 2021) doesn't open for me, and I need to work in unity 6 anyway when contributing. I've moved the project to the newest unity 6.0 LTS.
All changes were made either automatically by unity during the switch, or by me updating all the packages used by the project. No code changes are present, as none were needed.

Worth noting

In package.json I switched the "unity" property (which indicates the minimal unity version the package is guaranteed to work with) to unity 6000.0, since while it doesn't prevent anyone from installing reflex on older versions, it indicates that we promise nothing for non-lts versions of unity. Depending on the support goals it might be worth reverting the commit, or putting an older version there.

How Has This Been Tested?

Edit mode and play mode tests all pass. I see no errors anywhere, although it's probably worth to double check before merging. I've had issues on the 2021 version on some machines while I didn't have them on others - something similar could happen on unity 6

Checklist:

• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• I have checked that Reflex.GettingStarted still runs nicely