Provide graph of weekly usage core statistics by major version

Problem/Motivation

The graph for weekly core usage statistics → has become hard to read, because there is a layer in the graph for every minor version of Drupal 8 and later.

Steps to reproduce

Visit this page → and observe the graph.

Proposed resolution

Provide a graph of weekly usage core statistics by major version instead of the current graph, where there is a layer in the graph for every minor version.

Remaining tasks

Do it.

User interface changes

A graph that is simpler to read becomes visible to visitors of that page on Drupal.org.