Form Style Updates: Helper Text Color

The helper text under input fields have a color of #919191

The Form Sketch (https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/forms-spec-previews/) styles the color as #707070

We need to update the text color to #707070.

Note:

This color is coming from a bootstrap class .text-muted and bootstrap included !important in this style. Updating the text color may require additional configurations.