Menu: Revisit design of menu group titles

[lboehling]

This is a follow up issue from our 3/31 design share --

@asumihasan mentioned that UXers on different tenants in HCC shared feedback that the group titles in our menu component look clickable, leading to frustration when people can’t interact with them.

This is due to our secondary and regular text colors being very similar shades. (note: Our current secondary text color is the lightest shade of gray that we can use for text that still meets a 4.5:1 contrast ratio with our darkest background color [gray-20])

We should explore alternate ways of styling the grouped menu items to clarify the difference. One obvious solution is to add weight to the title, which would be consistent with text treatment in form labels and grouped navigation titles.