Only recommand for php-sodium on >= PHP 7.4

This is because php-sodium will solve the missing PASSWORD_ARGON2I
constant problem only on >= php 7.4, previously argon2 wasn't part of
the standard extension and was disabled on Centos/RHEL.

So installing php-sodium on php 7.2 for example wouldn't hide the
message. Tested on Fedora with php 7.4, Centos 7 with php 7.2,
Centos 8 with php 7.2 and openSUSE with php 7.4.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>