This patch is motivated by the desire to make the output of getchaintxstats more useful and optimized for applications to consume and render the data.
Firstly, this data is already available to the RPC, no additional work is done. Currently additional RPC calls will be needed to look up the height of the final block in the window or the block height that began the window.
By adding the block height of the final block in the window, the JSON is "self-contained" and applications can calculate the exact block height range of the window with no additional RPC requests.
For example, a web application which wants to render historical information for getchaintxstats RPC on various window sizes might call the RPC with various window lengths, once per day, and store the JSON results somewhere. Because the final block height of each dataset is included, it's no extra work to determine the exact block window range of each JSON response.

Backport of Core PR16695