[VSA] Add Warning Banner about behavior change in calculation in 15.0

Release notes

Problem to solve

We're changing how the date filter works. Instead of filtering by creation time, we will filter by end event time. Users who were monitoring the VSA metrics (and relied on it) will see completely different figures once this change is rolled out.

We should warn the users about this in advance

Proposal

Add a notification banner with the following text:

Value Stream Analytics are currently calculated by the start time. There is an [epic](https://gitlab.com/groups/gitlab-org/-/epics/6046) that will change the Value Stream Analytics calculation to end time.

This banner should be added to both group level VSA.

(project level VSA requires license checks - this is out of scope of this issue)

Also add a section to the docs that explains the new calculation change in %15.0

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