[Proposal] Remove `mujoco-py` based environments at May 2025 about gymnasium

[pseudo-rnd-thoughts]

While I think its a reasonable suggestion to move the mujoco-py environments into another repo such that users can still access them if necessary, gymnasium-robotics would be the most obvious location.

But I don't totally understand the issue for keeping them in the repo?
The problem with cpython is a semi-permentant issue with mujoco-py no matter the location, therefore, this is more a problem for users than the repo imho.

The biggest motivation would be to encourage users to migrate to v4 / v5 through making them no longer part of the easy install

[ReHoss]

+1 since mujoco-py triggers exceptions like below:

Exception: 
You appear to be missing MuJoCo.  We expected to find the file here: /home/mpanchen/.mujoco/mujoco210

Also appearing in #896

[RogerJL]

Prior to release of 1.0 would be the perfect time to remove this - no API promises on alfa/beta...
If someone needs backward compatibility they can use old git version of Gymnasium IMHO.