WIP: feat(form input): drop underlying BootstrapVue component

The BootstrapVue component on which GlFormInput relies doesn't bring any value to our own component anymore and can be dropped to reduce the overhead and to make it easier to style the input without having to deal with Bootstrap class being applied by BootstrapVue

This MR is motivated by the discussion here: gitlab!19777 (comment 242235551) We noticed that the stylesheets order is not the same in Storybook and in GitLab, which made us overlook an integration issue that popped up when we tried to upgrade gitlab-ui in GitLab. This should fix it because the .form-control class won't be applied to our form input component anymore.