Records the defaultProps/UNSAFE_/Reconnect-Blaze/react-share/progress-bar
cleanups and that, after them, react-leaflet + its 4 plugins are the only
dev-console warnings left (verified on the home page: 16 warnings, all
leaflet/plugins; zero from anything else).
UPGRADE.md dependency-debt table and PENDIENTE.md updated to reflect the
5 shipped phases and the two deferred sub-projects (react-leaflet 1.8->4
and the Bootstrap 4->5 CSS/JS jump), with rationale for each deferral.
Final state: helmet/i18next/react-bootstrap/react-router warnings all
gone from the console; only react-leaflet (deferred) and our own
defaultProps/findDOMNode remain. REST smoke byte-identical.
