Update Plan widget container backgrounds to correct Pajamas $gray-10

Context

The current Plan widgets are using our deprecated gray #fafafa causing inconsistencies as we add in new components.

Solution

Change any instances of #fafafa to our $gray-10 (#f5f5f5) anywhere it appears within work items like issues and epics. This currently appears in the:

• Linked/related items widget
• Related epics widget
• Epic tree widget
• MR widget
• Anything else?