AIRO-0000 Remove the UNITY_2020_1_OR_NEWER & UNITY_2020_1 defines

[K-Tone]

... and paths leading to them. We have ArticulationBody support in any Unity release since 20.3 LTS, no one in robotics should be using the Rigidbody path instead.

This is a preliminary PR, intended to land after discussion with Vidur.

Proposed change(s)

Describe the changes made in this PR.

Useful links (GitHub issues, JIRA tickets, forum threads, etc.)

Provide any relevant links here.

Types of change(s)

• [ ] Bug fix
• [ ] New feature
• [x] Code refactor
• [ ] Documentation update
• [ ] Other (please describe)

Testing and Verification

Please describe the tests that you ran to verify your changes. Please also provide instructions, ROS packages, and Unity project files as appropriate so we can reproduce the test environment.

Test Configuration:

• Unity Version: [e.g. Unity 2020.2.0f1]
• Unity machine OS + version: [e.g. Windows 10]
• ROS machine OS + version: [e.g. Ubuntu 18.04, ROS Noetic]
• ROS–Unity communication: [e.g. Docker]

Checklist

• [x] Ensured this PR is up-to-date with the dev branch
• [x] Created this PR to target the dev branch
• [ ] Followed the style guidelines as described in the Contribution Guidelines
• [ ] Added tests that prove my fix is effective or that my feature works
• [ ] Increased the test coverage criteria by 3%
• [ ] Updated the Changelog and described changes in the Unreleased section
• [ ] Updated the documentation as appropriate

Other comments

[unity-cla-assistant]

All committers have signed the CLA.

[JuozasK]

One thing I just noticed, URDF Robot script still says "All rigidbodies" in the editor. We should rename that to "All Articulation bodies"

[vidurvij-Unity]

Please add the relevant jira ticket in the description of the pull request.

[K-Tone]

Jira link: https://jira.unity3d.com/browse/PHYS-135