Instances of `gray` as a SCSS color

The following discussion from gitlab-ce!25266 should be addressed:

• @annabeldunstone started a discussion:

  Why are we using gray? 😱

Usage of gray as a value can be found in:

• blocks.scss
• commits.scss

We should be using one of our gray variables found in variables.scss

Related commit- https://gitlab.com/gitlab-org/gitlab-ce/commit/33dee95eab597dd85656c44968dad38b8f8716d7