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>

README.md

@@ -348,7 +348,7 @@ project.
 
 ### Release Team
 
-Releases of Node.js and io.js will be signed with one of the following GPG keys:
+Node.js releases are signed with one of the following GPG keys:
 
 * **Chris Dickinson** <christopher.s.dickinson@gmail.com>
 `9554F04D7259F04124DE6B476D5A82AC7E37093B`
@@ -383,8 +383,7 @@ gpg --keyserver pool.sks-keyservers.net --recv-keys 56730D5401028683275BD23C23EF
 See the section above on [Verifying Binaries](#verifying-binaries) for details
 on what to do with these keys to verify that a downloaded file is official.
 
-Previous releases of Node.js have been signed with one of the following GPG
-keys:
+Previous releases may also have been signed with one of the following GPG keys:
 
 * **Isaac Z. Schlueter** <i@izs.me>
 `93C7E9E91B49E432C2F75674B0A78B0A6C481CF6`