remove openai branding, update gym terminology with gymnasium

[acxz]

follow up of #467
address #453

cc @cameronangliss

no logic change whatsoever, just renaming.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.39%. Comparing base (f458350) to head (1cef425).
Report is 103 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
+ Coverage   83.38%   85.39%   +2.01%     
==========================================
  Files          39       42       +3     
  Lines        3918     4417     +499     
==========================================
+ Hits         3267     3772     +505     
+ Misses        651      645       -6     

[cameronangliss]

Good call @acxz, this looks good to me at a glance. There seems to be something wrong with the CI (not due to your changes, it seems to be in other PRs too) so we should wait until that is fixed first.

[hsahovic]

Thanks @acxz - this looks good

[hsahovic]

@acxz Unfortunately it looks like your PR conflicts which changes that I just merged - i didn't realize it was the case. Can you rebase and fix the conflict?
I can also fork your branch and do it if you prefer.

[acxz]

rebased