Change font weight and color of bolded texts in MR widget

This issue is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/33095

Design/Proposal

Change this:

to this:

• should use color: rgba(0,0,0,.85); or equivalent
• should use font-weight: 600;

cc: @victorwu (making the https://gitlab.com/gitlab-org/gitlab-ce/issues/33095 more readable)