Text Changes: Replace "webmaster" with "site admin" in `register_new_user()`.

Follow-up to [48049], accidentally reverted in [48059].

Props kebbet.
See #50394.
Built from https://develop.svn.wordpress.org/trunk@48066


git-svn-id: https://core.svn.wordpress.org/trunk@47833 1a063a9b-81f0-0310-95a4-ce76da25c4cd