Adds gym.Env to DirectMARLEnv for Gymnasium v1.0.0

[kellyguo11]

Description

Due to recent release of Gymnasium 1.0.0, this now becomes the Gymnasium version that gets installed when running the installation process for Isaac Lab. Gymnasium 1.0.0 requires all environments to be a subclass of gym.Env when using the make interface. This change makes the DirectMARLEnv a subclass of gym.Env to satisfy this requirement.

Fixes #1002

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• 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
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there