Update to Unity 2021.3

[Caeden117]

This PR updates the Unity version of ChroMapper to Unity 2021.3.32f1.

This Unity update brings updates to several packages, as well as a switch to .NET Standard 2.1 so newer features of .NET can be used in the project. The change to .NET Standard is important for future additions I want to make to the project (AudioSync, the BPM detection library I wrote, heavily relies on Span<T>, which was added in .NET Standard 2.1)

TODO:

• Investigate: Input System update might've fucked up key rebinding

[XAce1337manX]

Bonus: I believe this may fix #416.

[Caeden117]

SortedSet<T>.GetViewBetween() crashing issue has been fixed by completely getting rid of SortedSet<T> 👍