Secondary text color contrast

gl-text-gray-400 is used for secondary text and lacks sufficient contrast (3.5:1) on a white background to satisfy 4.5:1 contrast required by WCAG 1.4.3: Contrast (Minimum) (Level AA).

Current usage of gl-text-gray-400: https://sourcegraph.com/search?q=context:global+repo:gitlab.com/gitlab-org/.*+%28gl-text-gray-400%7C%24gray-400%29&patternType=regexp&sm=1&groupBy=repo

Suggestion to use gl-text-secondary (grey-500) to ensure consistency and have a contrast score of 4.67:1.

See: !139334 (merged) and https://design.gitlab.com/product-foundations/color#accessibility