Replace append-bottom-20 CSS class

Summary

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

We should move the append-bottom-20 class to the gitlab-ui utility class of gl-mb-5 or gl-mb-7 depending on the design

The file list that contain the aforementioned class is listed below

File	converted
app/views/devise/unlocks/new.html.haml	✅
app/views/devise/shared/_signup_box.html.haml	✅
app/views/devise/shared/_experimental_separate_sign_up_flow_box.html.haml	❌ file removed
app/views/devise/confirmations/almost_there.haml	✅
app/views/projects/settings/_general.html.haml	✅
app/views/admin/runners/show.html.haml	✅
app/views/groups/settings/_general.html.haml	✅
app/assets/javascripts/snippets/components/show.vue	✅
app/assets/javascripts/snippets/components/snippet_blob_edit.vue	✅
app/assets/javascripts/snippets/components/edit.vue	✅
app/assets/javascripts/clusters/components/remove_cluster_confirmation.vue	✅
app/assets/javascripts/boards/components/board_form.vue	✅
app/assets/javascripts/diffs/components/tree_list.vue
app/assets/stylesheets/framework/common.scss
ee/app/views/shared/promotions/_promote_mr_features.html.haml
ee/app/views/trial_registrations/_signup_box.html.haml
ee/app/assets/javascripts/group_member_contributions/components/app.vue
ee/app/assets/javascripts/add_gitlab_slack_application/components/add_gitlab_slack_application.vue
ee/app/assets/javascripts/subscriptions/new/components/order_summary.vue
ee/app/assets/javascripts/geo_nodes/components/app.vue
ee/app/assets/stylesheets/pages/promotions.scss

Risks

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