Drop support for UE4 4.18 4.19 4.20 and Pin to 4.21

[ProfFan]

4.21 is the current release. 4.18 works well but 4.19 and 4.20 are horribly broken in rendering and physics.

We should just focus on solving bugs that appear in 4.21 only.

[ProfFan]

Now we have 4.22 Released. UE4 updates so fast.

[lexer21]

Hello,
Which is the current supported version? I've tried opening it in 4.22 but I failed to rebuild the plugins.

[ProfFan]

Currently the codebase is not in usable state. I do not current have time for development as I am in my 1st year of grad school :)

In case you still want to have a try, I developed on 4.18, ported for 4.20, but 4.20 got many bugs. In the process of porting to 4.20 I got grad school so it is kind of abandoned. Recommend you to go to https://github.com/robcog-iai/UROSBridge to see the folks' work. They are doing better than mine :)