Set the maximum velocity to 15km/h

[takayuki5168]

Checklist

• I've read the contribution guidelines.
• I've searched other issues and no duplicate issues were found.
• I've agreed with the maintainers that I can plan this task.

Description

Currently, the maximum velocity is 40km/h by default. We aim to set it to 15km/h by default.

Part of:

• Switch to obstacle cruise planner and enable AEB (autonomous emergency braking) #4520

Purpose

Based on the following discussion, in order to use obstacle_cruise_planner, setting 15km/h by default is required.
https://github.com/orgs/autowarefoundation/discussions/4052

Possible approaches

Set the maximum velocity to 15km/h

Definition of done

The maximum velocity is set to 15km/h