Fix Exec Dashboard time period for 3 months ago

What does this MR do and why?

On the Exec dashboard we're calculating metrics based on constant time periods, one of which is THREE_MONTHS_AGO. Turns out this one in particular was defined incorrectly, so this MR is to fix the end bound.

Before this change, the time period contained all the data from the previous 2nd + 3rd month. This will limit it to just the 3rd month as expected.

Screenshots

The difference is minimal, but the % change in the rightmost column changes alongside the time period fix. Aside from that, there are no visible changes

Before	After

How to set up and validate locally

1. Seed cycle analytics data (and optionally DORA metrics data)
2. Enable the Exec dashboard

   Feature.enable(:group_analytics_dashboards_page)

3. Visit the exec dashboard: http://gdk.test:3000/groups/${YOUR_GROUP}/-/analytics/dashboards

MR acceptance checklist

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