Resolve work item sticky header spacing issue

What does this MR do and why?

The work item sticky header was not proper in fluid layout. Fix the spacing issue in fluid layout.

This applies to work item tasks, objectives, key results and epics.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Screen Recording 2024-03-26 at 4.17.32 PM.mov	Screen Recording 2024-03-26 at 4.14.48 PM.mov

How to set up and validate locally

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).
2. Enable Epic work items FF - Feature.enable(:namespace_level_work_items)
3. Enable FF - Feature.enable(:work_items_beta)
4. Enable fluid layout from Profile > Preferences > Select Fluid in Layout width section

Steps to verify objectives/key results/tasks:

1. Login with any user
2. Go to any Project > Issues > List
3. Create an new Objective under New issue split button
4. Go to Objective detail page
5. Scroll downwards to make sticky header appear
6. Check if it renders properly

Steps to verify epics:

1. Login with any user
2. Go to any Group > Epic
3. Change the URL from /epics to /work_items e.g. http://127.0.0.1:3000/groups/gitlab-org/-/epics > http://127.0.0.1:3000/groups/gitlab-org/-/work_items
4. Create an new Epic work item
5. Go to the Epic work item detail page
6. Scroll downwards to make sticky header appear
7. Check if it renders properly

Related to #441619 (closed)