Lighten dashboard icons color to balance visual weight

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Problem

The icons in the Issues and Merge Requests dashboards are visually too strong.

Solution

Lighten the icon color to use $gray-500 (#666666) instead of currentColor, which is currently $gray-900 (#303030).

Before After
CleanShot_2020-09-24_at_10.52.50_2x TBD
Edited by 🤖 GitLab Bot 🤖