fix(mpc): change the way to calculate target steering rate

[tkimura4]

Description

Hotfix to beta/v0.10.3
autowarefoundation#5320

(TIERIV INTERNAL LINK https://star4.slack.com/archives/C03QW0GU6P7/p1697445979466779 )

（MPCの修正：これがないとフィルターに常に引っかかるが、走行挙動には変化なし）

Related links

Tests performed

none

Notes for reviewers

Interface changes

none

Effects on system behavior

MPCが制限値を超えてステア出力されてしまう挙動が修正される　

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