InvertedPendulumMPCControl: fixed deprecated waring about using * for…

[ksvbka]

Reference issue

What does this implement/fix?

Improve inverted_pendulum_mpc_control.py:

• Fix cvxpy warning about deprecated matrix multiplication syntax

This use of ``*`` has resulted in matrix multiplication.
Using ``*`` for matrix multiplication has been deprecated since CVXPY 1.1.
    Use ``*`` for matrix-scalar and vector-scalar multiplication.
    Use ``@`` for matrix-matrix and matrix-vector multiplication.
    Use ``multiply`` for elementwise multiplication.

Additional information

CheckList

-[] Did you add an unittest for your new example or defect fix?
-[] Did you add documents for your new example?
-[] All CIs are green? (You can check it after submitting)

[AtsushiSakai]

LGTM. Thanks