I think you are stand up for informing us and adding so much information here so someone like myself who is not very skilled can learn and hopefully help someone else with this down the road.
Thank you.

The problem is resolved.

In my case, the PHP_VERSION_ID constant returns 70133, indicating version 7.1.33, while the php -v command indicates version 8.1.7.

This suggests an inconsistency between the PHP version used by the web server and the one used in the command line. To have version 8.1.7 in my PHP scripts, I had to ensure that the PHP module used in my web server is configured to use this version. I made sure that the PHP module for Apache is configured to use version 8.1.

Here are the commands I executed:

sudo a2dismod php7.1
sudo a2enmod php8.1
sudo systemctl restart apache2

For any useful purpose, here is the code of the file platform_check.php that produces this error:

<?php

// platform_check.php @generated by Composer

$issues = array();

if (!(PHP_VERSION_ID >= 80100)) {
    $issues[] = 'Your Composer dependencies require a PHP version ">= 8.1.0". You are running ' . PHP_VERSION . '.';
}

if ($issues) {
    if (!headers_sent()) {
        header('HTTP/1.1 500 Internal Server Error');
    }
    if (!ini_get('display_errors')) {
        if (PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg') {
            fwrite(STDERR, 'Composer detected issues in your platform:' . PHP_EOL.PHP_EOL . implode(PHP_EOL, $issues) . PHP_EOL.PHP_EOL);
        } elseif (!headers_sent()) {
            echo 'Composer detected issues in your platform:' . PHP_EOL.PHP_EOL . str_replace('You are running '.PHP_VERSION.'.', '', implode(PHP_EOL, $issues)) . PHP_EOL.PHP_EOL;
        }
    }
    trigger_error(
        'Composer detected issues in your platform: ' . implode(' ', $issues),
        E_USER_ERROR
    );
}

Yes php -v gives:

PHP 8.1.7 (cli) (built: Jun 25 2022 08:12:27) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.7, Copyright (c) Zend Technologies
with Zend OPcache v8.1.7, Copyright (c), by Zend Technologies

And sudo update-alternatives --config php gives:

Selection    Path             Priority   Status
------------------------------------------------------------
* 0            /usr/bin/php8.1   81        auto mode
  1            /usr/bin/php7.1   71        manual mode
  2            /usr/bin/php7.2   72        manual mode
  3            /usr/bin/php8.1   81        manual mode

Well, since it's not the first time that people here are not aware of the difference between the PHP command-line version and the PHP webserver version, please allow the question:
Does php -v really gives 8.1.7 on your command-line?