MuJoco-v5 environments replace ndarray.flat.copy() with ndarray.flatten()

[Kallinteris-Andreas]

Description

replaces ndarray.flat.copy() with ndarray.flatten() which have the same behavior (https://stackoverflow.com/questions/77579701/does-ndarray-flat-copy-and-ndarray-flatten-do-the-same-thing) and should in theory be slightly faster (though I could not measure an overall speedup)

motivation

MJX & jax does not support array.flat jax-ml/jax#8046, but they support array.flatten()

validation

in the repo: https://github.com/Kallinteris-Andreas/gymnasium-changes-validation/tree/main/flatten
I validated that over 1 mil steps the behavior is identical for all environments
and observed no measurable runtime slowdown

Type of change

• Effectively no change
• 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