Use MuJoCo Menagerie model for `gen3_mj_description` #272

Job	Run time
Code style	56s
Coverage	3m 9s
Loader: idyntree with Python 3.9	2m 5s
Loader: mujoco with Python 3.9	39s
Loader: pinocchio with Python 3.9	3m 25s
Loader: pybullet with Python 3.9	2m 46s
Loader: robomeshcat with Python 3.9	5m 41s
Loader: yourdfpy with Python 3.9	2m 47s
CI success	1s
	21m 29s

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