Replace prepend-left-20 CSS class

Summary

As part of our efforts to move to gitlab-ui based utility classes in #217418

We should move the prepend-left-20 class to the gitlab-ui utility class of gl-ml-5 or gl-ml-7 depending on the design

The file list that contain the aforementioned class is listed below

File	Converted
app/views/projects/network/show.html.haml	✅
app/views/admin/hooks/_form.html.haml	✅
app/views/shared/web_hooks/_form.html.haml	✅
app/assets/stylesheets/framework/common.scss	✅

Improvements

Risks

Not every single file will use the same class, it is neccessary to request a UX review for each change being made