Use GlTable instead of `ci-table`

Inside the component app/assets/javascripts/jobs/components/manual_variables_form.vue we are using a native html element with CSS classes to build a table. We instead should use the component GlTable and replace the input's nested in the table with the proper gitlab-ui components.

https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/jobs/components/manual_variables_form.vue#L108

Note: While you're at it, it would be helpful to replace the CSS classes in this component with gitlab-ui utility classes 👍

Edited by Payton Burdette