Fixes rigid object's root com velocities timestamp check

[ori-gadot]

Description

The rigid body data properties root_com_vel_w, root_com_pos_w, and root_com_ang_vel_w currently check the private state field's timestamp before updating their state. However, the issue lies in the fact that these COM state properties are incorrectly checking the link state field's timestamp instead of the COM's own state.

Fixes # (issue)
Replacing root link state with root com state at these properties.

Type of change

• Bug fix (non-breaking change which fixes an issue)

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