doc: drop "and io.js" from release section

Reword the language around releases and signing keys to make it clear
that new releases are all branded "Node.js."

PR-URL: https://github.com/nodejs/node/pull/11054


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>