Add PHP 8 support on Apache2 conf

[ Impact ]

In Focal, the apache2 configuration file shipped with phpmyadmin sets 2 php.ini directives through "php_admin_value": upload_tmp_dir and open_basedir.

After the PHP 8 transition an unintentional regression was introduced and these directives are no longer being set. As a consequence, the temporary phpmyadmin directory at /var/lib/phpmyadmin/tmp is no longer being used as the temporary path for uploads. Instead, the system temp path is used.

Moreover (and more relevant), open_basedir is set to NULL, meaning that the php process running phpmyadmin can easily read files outside of the phpmyadmin package scope.

However, due to the notes in https://www.php.net/manual/en/ini.core.php#ini.open-basedir, it is arguable whether this should be treated as a security threat [1,2,3,4].

[1] https://externals.io/message/105606
[2] https://externals.io/message/115406
[3] https://wiki.php.net/security#not_a_security_issue
[4] https://www.php.net/manual/en/ini.core.php#ini.open-basedir

Finally, this regression could also cause crashes when users set open_basedir globally for apache2 somehow, as discussed in https://github.com/phpmyadmin/phpmyadmin/issues/18299.

[ Test Plan ]

 * From a fresh ubuntu installation:
   - install phpmyadmin and libapache2-mod-php
   - browse to http://localhost/phpmyadmin and verify that the UI is working as expected (i.e., you see a login page).
   - append "php_admin_value open_basedir ." to /etc/apache2/conf-enabled/phpmyadmin.conf
   - restart Apache2
   - browse to http://localhost/phpmyadmin again and verify that the UI is no longer working as expected. You now see a blank page and /var/log/apache2/error.log has an error like

PHP Fatal error: Uncaught Error: Failed opening required 'PhpMyAdmin/MoTranslator/autoload.php' (include_path='.:/usr/share/php')

    - Perform all the steps above, now with a phpmyadmin package with the proposed fix
    - verify that the error no longer occurs.

ALTERNATIVELY:

Append the following code snippet to the beginning of /usr/share/phpmyadmin/index.php, right after the imports (i.e., "use ...")

$secret_file = '/etc/hosts';
$secret = file_get_contents($secret_file);
if ($secret) {
 error_log($secret);
}

Restart apache2 and browse to http://localhost/phpmyadmin.

Now, check /var/log/apache2/error.log.

Affected systems will contain en entry with the contents of /etc/hosts, while patched systems will contain

file_get_contents(): open_basedir restriction in effect....
file_get_contents(/etc/hosts): Failed to open stream: Operation not permitted ...

[ Where problems could occur ]

Some users may be relying on the upload_tmp_dir being set to /tmp to perform additional verification or collect metrics during phpmyadmin runtime. This would create a regression for those users.

Moreover, users could be extending phpmyadmin in unusual ways by relying on the fact that open_basedir is not set (being able to access the whole filesystem through its php process). This would also create a regression for those users.

[ Other Info ]

This issue has been fixed from lunar and on.

See: (https://github.com/phpmyadmin/phpmyadmin/issues/18299
See: https://bugs.launchpad.net/ubuntu/+source/phpmyadmin/+bug/2013402