Remove $list-border SCSS variable

Remove $list-border (rgba(0, 0, 0, 0.05)) SCSS variables from _variables.scss and replace usage with appropriate GitLab UI design token or variable.

This should use a color value from GitLab UI so that the color is updated between modes. Either a SCSS variable like $gray-50, or a semantic token like var(--gl-border-color-subtle) (or gl-border-subtle utility class).