Branch release 21 is stale and the tutorial recommends this branch

[RishiMalhotra920]

Branch release 21 contains assets that are stale and scripts that don't compile correctly. The 3DBall Agent in this branch still uses scripts with Barcudda. Assets/ML-Agents/Examples/3DBall/TFModels/3DBall.nn is no longer supported it seems. When I try to drag and drop it into the agent model field, it doesn't work.

The tutorial here: https://github.com/Unity-Technologies/ml-agents/blob/release_21_docs/docs/Installation.md recommends using this branch version. I had to manually find the discrepancies between this branch and #6030 and make the fixes.

To fix, either upgrade branch release 21 or reference a different release in the tutorial