Update Piper MJCF path and Update MuJoCo Menagerie Commit Hash

[jonzamora]

Hi,

This PR updates the Piper MJCF path to use piper.xml instead of scene.xml. This matches the convention used by other robot arms in this project, which load only the arm with the MJCF path instead of the scene.xml. I also updated the MuJoCo Menagerie Commit Hash to the latest version 🚀

[coveralls]

Pull Request Test Coverage Report for Build 15596425077

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.968%

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Totals
Change from base Build 15558924677:	0.0%
Covered Lines:	1109
Relevant Lines:	1132

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💛 - Coveralls

[stephane-caron]

Thank you for this update 👍