CLEANUP: stats: use modern DOCTYPE tag

CLEANUP: stats: use modern DOCTYPE tag

Switching the stats page doctype to the modern standard is shorter and
less complex, and is the recommended doctype by current HTML standard.
It makes it clear that we do not want to run in quirks mode. More information below.

Quirks mode: https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode
HTML Standard: https://html.spec.whatwg.org/multipage/syntax.html#the-doctype