Update wi hierarchy widget styles

What does this MR do and why?

Update work item hierarchy widget styles for consistency

• Updates Add button to small variant
• Update paddings

Padding change details: #369895 (closed)

Button changes details: #369925 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1.Enable the following feature flags

Feature.enable(:work_items)
Feature.enable(:work_items_hierarchy)

1. Visit an issue page containing tasks

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #369925 (closed)