Look in wp-includes/version.php

/**
 * The WordPress version string
 *
 * @global string $wp_version
 */
$wp_version = '2.8.4';

Most likely you have installed both versions of PHP,

1. the PHP version provided by default by the AWS Ubuntu 18,
2. the PHP version provided by the third-party repository deb.sury.org.

However, whichever version is used by your Web server, is the version that is configured in there. For example, if you are using nginx, then check the configuration file of the server block, where it says the following. You need to find the correct Unix socket for the third-party PHP package, and replace /var/run/php/php7.0-fpm.sock with the proper Unix socket. Finally, restart the Web server.

    # pass PHP scripts to FastCGI server
    #
    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
    #
    #   # With php-fpm (or other unix sockets):
        fastcgi_pass unix:/var/run/php/php7.1-fpm.sock;
    #   # With php-cgi (or other tcp sockets):
    #   fastcgi_pass 127.0.0.1:9000;
    }

On a system where

• you can't change the /usr/bin/php symlink
• you can't change the PATH to point to a different version (because the php executables don't reside in distinct /lib/ directories)
• WP_CLI_PHP has no effect

(like my Arch Linux with php(8), php7 installed from extra and wp-cli installed from AUR. I'm using php7 vs php8 here, but this should work for any versions.)

…a workaround may be to call the wp phar executable with php7 cli:

whereis wp
# /usr/bin/wp

php7 /usr/bin/wp cli info
# PHP binary:     /usr/bin/php7
# PHP version:    7.4.25

for convenience you can add a bash alias in your .bashrc:

alias wp-php7='php7 /usr/bin/wp'
# or override wp altogether
alias wp='php7 /usr/bin/wp'