Stats: drop the per-widget "Number of results" setting (#51378)

* Stats: drop the per-widget "Number of results" setting

Widgets render into fixed-height tiles and only show the rows that fit, so
the control changed nothing visible while quietly changing how much data was
fetched and how large the CSV export was. Every list widget now requests the
shared WIDGET_ROW_LIMIT; report pages keep their pagination control for
"show me more rows".

Seventeen of the eighteen widgets lose their settings drawer entirely — the
host hides the trigger once no low-relevance attribute is left. Email
breakdown keeps its drawer for the view/metric selectors on the fixed
post-detail page.

* docs: simplify widget row limit guidance

* refactor: inline the shared widget row limit

* fix: drop a dead row-count clamp and a vacuous archives test

`Math.min( WIDGET_ROW_LIMIT, 30 )` had two constant operands once the widget
stopped reading a user-set row count, so it always returned 10 while reading
like live logic. Pass the limit straight through and keep the endpoint's 1-30
range in the comment, which is the part still worth knowing.

The archives test lost its subject when `max: 0` went away: with two groups
and a fixed limit of 10 no capping path runs, so it asserted only what the
test above it already covers, with fewer assertions.

* test: wait for the composite item list before driving the date filter with keys

* docs: scope the widget row-count rule to Stats widgets

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/32348882841

Upstream-Ref: Automattic/jetpack@8b11c25

Author: jeherve