Update Pusher-v5 model to one that is compatible with mujoco>=3

[Kallinteris-Andreas]

Description

increased the weight of the object to 7.85398163e-06 kg from 1.30899694e-08 kg (500x increase)

Fixes #950

testing & validation was done using: https://github.com/Kallinteris-Andreas/Gymnasium-Pusher-v5-model-validation

credits

@Kallinteris-Andreas

and thanks to @HernandezEduin, for initial testing

Type of change

• Documentation only change (no code changed)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

Could you add a warning to the version history of v4

[Kallinteris-Andreas]

There is now a warning for v4 and mujoco>=3 in the documentation for users using gymnasium<=1.0.0a1, for future users are not about to instantiate the environment with an incompatible versions for mujoco

[pseudo-rnd-thoughts]

Thanks for the changes