doc: remove outdated ARM information from release guide

We no longer produced ARMv6 builds, and the ARMv7 builds are now
cross compiled and are as quick as builds on the other platforms.

PR-URL: https://github.com/nodejs/node/pull/39544


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Danielle Adams <adamzdanielle@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>