Improve Value Stream Analytics message on empty pages

What does this MR do and why?

While investigating this internal customer ticket, I found no "magic" cut-off number that would imply our "not enough data" message.

Thus, this MR clarifies that message.

Screenshots or screen recordings

How to set up and validate locally

1. Normal GDK start
2. Create an empty group and a Value Stream with 1 or 2 stages
3. Add a project & cycle through a few issues and MRs, according to the VSA stages

MR acceptance checklist

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