Use `<gl-form-checkbox />` in Web IDE commit option for creating an MR

This is a follow up to https://gitlab.com/gitlab-org/gitlab-ce/issues/57482 and gitlab-ui#132 (closed). Since gitlab-ui#132 (closed) is slated for 11.11 and was not finished at the time of https://gitlab.com/gitlab-org/gitlab-ce/issues/57482, a normal <input type="checkbox" /> was used.

This issue is to go back and replace it once the new Checkbox is finished.