Fix VSD column names not matching the date range

What does this MR do and why?

Fixes a bug where on the first & last day of the month, the month name used as the column header did not match the date range used for the column. This was primarily caused by nMonthsBefore being used to transition to a month with shorter days (ex. March 31 > Feb 28 actually returns a day in early March, since Feb 31 does not exist).

I cleaned up the logic a bit and added a new test to avoid future regressions.

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Before	After

How to set up and validate locally

1. Navigate to the VSD for any group:

http://gdk.test:3000/groups/flightjs/-/analytics/dashboards/value_streams_dashboard

2. Mock the current date by returning a custom Date() here

export const generateValueStreamDashboardStartDate = () => {
  return new Date('2020-03-31');
};

3. Validate that the table shows the correct column headers

Related to #439901 (closed) and #430043 (closed)