Remove form-horizontal class where applicable

What does this MR do?

Replaces .form-horizontal with .form-group.row or removes it entirely (where applicable)

Screenshots (if relevant)

app/views/admin/application_settings/_terms.html.haml

_repository_mirrors_form.html.haml

app/assets/javascripts/ide/components/commit_sidebar/form.vue

app/assets/javascripts/pages/projects/wikis/components/delete_wiki_modal.vue

app/views/shared/members/_member.html.haml

Mobile:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

closes #46744 (closed)